dev, arm: Use the PS/2 framework in the Pl050 model

dev, arm: Use the PS/2 framework in the Pl050 model

The Pl050 KMI model currently has its own keyboard and mouse
models. Use the generic PS/2 interface instead.

Change-Id: I6523d26f8e38bcc8ba399d4d1a131723645d36c7
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/9767
Reviewed-by: Gabe Black <gabeblack@google.com>