arch-arm: We add PRFM PST instruction for arm

arch-arm: We add PRFM PST instruction for arm

Note current PRFM supports only PLD, but PST (prefetch for store) is
also important for latency hiding. We also bug fix in disassembler to
display prfop correctly.

Change-Id: I9144e7233900aa2d555e1c1a6a2c2e41d837aa13
Signed-off-by: Yuetsu Kodama <yuetsu.kodama@riken.jp>
Reviewed-on: https://gem5-review.googlesource.com/c/13675
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>