ps2: Unify device data buffering

All PS/2 device currently implement various ad-hoc mechanisms to
handle multi-byte commands. This is error-prone and makes it hard to
implement new devices. Create a buffering mechanism in the base class
to avoid this.

Change-Id: If5638b0ab68decea8de7631ecead0a9ebad1547b
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/9765
Reviewed-by: Gabe Black <gabeblack@google.com>

ps2: Factor out PS/2 devices into their own subsystem

PS/2 devices are currently emulated both in the i8042 model and the
Arm KMI model. This is undesirable since it leads to code duplication.

This change introduces a common PS/2 device interface and factor out
the x86 keyboard and mouse model. A subsequent commit will implement
support for this interface in the Arm KMI model.

Change-Id: I440e83517fd9dce362fdc1676db477cc6eee5211
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/9762
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>