arch-power: Add fields for DS form instructions

This introduces the DS field used by DS form instructions
which specifies a signed integer immediate operand.

Change-Id: I0e7a77e7a63fce4e50b7941850c277f556e65724
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40891
Reviewed-by: Boris Shingarov <shingarov@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/power/types.hh

@@ -52,6 +52,7 @@ BitUnion32(ExtMachInst)
     // Immediate fields
     Bitfield<15,  0> si;
     Bitfield<15,  0> d;
+    Bitfield<15,  2> ds;
 
     // Special purpose register identifier
     Bitfield<20, 11> spr;