arch-arm: inform bootloader of kernel position with a register

Before the commit, the bootloader had a hardcoded entry point that it
would jump to.

However, the Linux kernel arm64 v5.8 forced us to change the kernel
entry point because the required memory alignment has changed at:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
commit/?h=v5.8&id=cfa7ede20f133cc81cef01dc3a516dda3a9721ee

Therefore the only way to have a single bootloader that boots both
pre-v5.8 and post-v5.8 kernels is to pass that information from gem5
to the bootloader, which we do in this patch via registers.

This approach was already used by the 32-bit bootloader, which passed
that value via r3, and we try to use the same register x3 in 64-bit.

Since we are now passing this information, the this patch also removes
the hardcoding of DTB and cpu-release-addr, and also passes those
values via registers.

We store the cpu-release-addr in x5 as that value appears to have a
function similar to flags_addr, which is used only in 32-bit arm and
gets stored in r5.

This commit renames atags_addr to dtb_addr, since both are mutually
exclusive, and serve a similar purpose, DTB being the newer recommended
approach.

Similarly, flags_addr is renamed to cpu_release_addr, and it is moved
from ArmSystem into ArmFsWorkload, since it is not an intrinsic system
property, and should be together with dtb_addr instead.

Before this commit, flags_addr was being set from FSConfig.py and
configs/example/arm/devices.py to self.realview.realview_io.pio_addr
+ 0x30. This commit moves that logic into RealView.py instead, and
sets the flags address 8 bytes before the start of the DTB address.

JIRA: https://gem5.atlassian.net/browse/GEM5-787
Change-Id: If70bea9690be04b84e6040e256a9b03e46710e10
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35076
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

configs/common/FSConfig.py

@@ -250,7 +250,7 @@ def makeArmSystem(mem_mode, machine_type, num_cpus=1, mdesc=None,
     if bare_metal:
         # EOT character on UART will end the simulation
         self.realview.uart[0].end_on_eot = True
-        self.workload = ArmFsWorkload(atags_addr=0)
+        self.workload = ArmFsWorkload(dtb_addr=0)
     else:
         workload = ArmFsLinux()
 
@@ -269,8 +269,6 @@ def makeArmSystem(mem_mode, machine_type, num_cpus=1, mdesc=None,
         if hasattr(self.realview.gic, 'cpu_addr'):
             self.gic_cpu_addr = self.realview.gic.cpu_addr
 
-        self.flags_addr = self.realview.realview_io.pio_addr + 0x30
-
         # This check is for users who have previously put 'android' in
         # the disk image filename to tell the config scripts to
         # prepare the kernel with android-specific boot options. That