arm: mmio emulation skeleton

arm: mmio emulation skeleton

This patch adds the necessary code for handling MMIO accesses. The trap
handler fills a mmio_access struct according to the fields in the ESR,
and passes it to all relevant sub-handlers.
If all return UNHANDLED, the access is considered invalid and the CPU is
put into failed mode.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
[Jan: switch to mmio accessors, use asm/mmio.h]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>