arch-arm: Read VMPIDR instead of MPIDR when EL2 is Enabled

arch-arm: Read VMPIDR instead of MPIDR when EL2 is Enabled

Trying to read MPIDR(_EL1) from EL1, should return the value of
VMPIDR_EL2 if EL2 is enabled. This patch is modifying the utility
function for reading MPIDR in order to match this behaviour for both
AArch32 and AArch64.

Change-Id: I32c2d4d5052f509e6e0542a5314844164221c6a3
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15617
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>