Disable atkbd0 and atkdbc0 in EC2 AMIs. This has the effect of skipping
the probing and attaching of the PS/2 mouse (not present on EC2) and
keyboard (emulated, but not accessible via EC2).

Note that we disable atkbd0 separately even though during device probing
it shows up as a child of atkbdc0; this is necessary because the device
is also initialized during the early console setup from hammer_time.

This change cuts the kernel boot time on an EC2 c5.4xlarge instance from
7259ms down to 4727 ms.

Approved by: re (marius)