ARM: kernel: introduce SoC abstraction

This introduces InterruptController and HardwareTimer classes to
handle the SoC specific implementations of timers and ints for
the ARM platform.

These could be improved and moved to a more 'generic' level once
we're confident they are 'good enough'.

NOTE: The OMAP timer implementation is fully untested and probably
      completely non-functional....
ARM: u-boot: Generate DTB and include in uImage
Build media add-ons for secondary arch.

Fixes #10049.