cpu: Get rid of auto return types in the PhysRegFile.

cpu: Get rid of auto return types in the PhysRegFile.

This is a C++14 feature, where we only support up to C++11 currently. It
also unnecessarily obfuscates what these functions are doing, since the
return type is a simple and fixed.

Change-Id: I7459ed885c3f006edbcecd4c6be9835d77dbbbae
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30434
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>