hardware_gen: Always specify kernel devices

The kernel device IRQs and Frame mappings generated by this script will
only come from nodes specified in the seL4,kernel-devices property of
the chosen node.  Previously these devices were inferred by the script
but this led to false matching and didn't support easily overriding
which devices to match under different configurations or across
different platforms.

Explicitly specifying which devices from the device tree will be used in
the kernel makes it easier to check which devices the kernel is actually
using and makes it easier to change on a per platform or per
configuration basis.