ps2: Unify constant names

Move ps2.hh to dev/ps2/types.hh and update the device models to
consistently use well-known constants from this header.

Change-Id: Iadfdc774495957beb82f3d341107b1e9232ffd4c
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/9770
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>