arch-riscv,dev: Explicitly set num CPUs on platform

Previously, the RISC-V devices queried the system object in
SimObject::init() for the number of CPUs and the number of threads.
However, the system object doesn't actually count the number of
CPUs/threads until it runs init(). Therefore, we've just been getting
lucky in the order that the SimObject init() functions were called.

This change instead decouples these two functions and makes the number
of CPUs/threads a parameter for the RISC-V interrupt devices. This
change also updates the example config script.

Change-Id: Ic4da5604156837cfeec05e58d188b42a02420de1
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49431
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

configs/example/riscv/fs_linux.py

@@ -185,6 +185,7 @@ system.bridge.ranges = system.platform._off_chip_ranges()
 system.platform.attachOnChipIO(system.membus)
 system.platform.attachOffChipIO(system.iobus)
 system.platform.attachPlic()
+system.platform.setNumCores(np)
 
 # ---------------------------- Default Setup --------------------------- #