arch-arm: serialize miscregs as a map

arch-arm: serialize miscregs as a map

This will prevent checkpoints from breaking on every miscreg addition.

Before this commit, miscregs were stored as an array:

[system.cpu.isa]
miscRegs=965 0 0 0 0 0 0 0 0 0 0 0 17895697 ...

and after this commit they are stored as a map:

[system.cpu.isa]

[system.cpu.isa.miscRegs]
cpsr=965
spsr=0
spsr_fiq=0
spsr_irq=0
spsr_svc=0
spsr_mon=0
spsr_abt=0
spsr_hyp=0
spsr_und=0
elr_hyp=0
fpsid=0
fpscr=0
mvfr1=17895697

JIRA: https://gem5.atlassian.net/browse/GEM5-661
Change-Id: I49999c7206bd9ac1cfb81297d45c8117ff8ae675
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36116
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>