arch-arm: Remove setters from SoftwareStep

arch-arm: Remove setters from SoftwareStep

Motivation:
Those helpers are used and meant to be used by the parent
(SelfDebug) class only. There is no point on exposing them to
the outer world. Better to make SelfDebug a friend class and to
allow it to access children's private data.

Change-Id: Ib945b1aa46742b90062ce7a5de563f164127075f
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/+/32774
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>