arch-arm: Correct the Ids and names of the PMU events

arch-arm: Correct the Ids and names of the PMU events

0x0C is the PC_WRITE_RETIRED event and 0x21 is the RetiredBranches.

Change-Id: I5f1173ff06f67b6a46e8a914c8acb9639edf67ec
Signed-off-by: Andriani Mappoura <andriani.mappoura@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26485
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>