arch-arm: Reorder arch/arm/registers.hh constants

arch-arm: Reorder arch/arm/registers.hh constants

This is putting some order in the constants definition, respecting
the description which divides:

* Constants Related to the number of registers
 (example: const int NumFloatRegs = 0)

from:

* Semantically meaningful register indices (to indicate special
  registers)
 (example: const int INTRLVREG0 = NumVecV8ArchRegs + NumVecSpecialRegs)

Change-Id: I1760b7f786b6f6becbe8ab445e65fc3fa17206cb
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23104
Tested-by: kokoro <noreply+kokoro@google.com>