Disable atkbd0 and atkdbc0 in EC2 AMIs. This has the effect of skipping

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)