dev: Fix i8042 device errors

dev: Fix i8042 device errors

The patch that added M5_FALLTHROUGH (5c41076bd7610 misc: Updates for gcc7.2
for x86) incorrectly added breaks to the i8042 device without implementing
the correct functions. This patch implements keyboard writes, but ignores
output writes.

Information on the PS2 controller can be found at
https://wiki.osdev.org/%228042%22_PS/2_Controller

Note: Without this patch Linux 4.14 won't boot.

Change-Id: I7de137b46cef00e6c1f1c14335cb52107cd7fe5b
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/7301
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>