ps2: Add VNC support to the keyboard model

ps2: Add VNC support to the keyboard model

Add support for keyboard input from the VNC server in the PS/2
keyboard model. The introduced code is based on the functionality in
the Arm PL050 KMI model.

Change-Id: If04a9713e5a15e2149d1a7471b999e3060d8ee7d
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/9763
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>