arm: IRQ handling skeleton

arm: IRQ handling skeleton

Since IRQs taken to HYP use a different vector, the trap handler needs to
be aware of the exit context. To this end, the patch adds an 'exit_reason'
field to the struct registers.
The structure is still passed to the dispatcher as a pointer to the stack,
but care must be taken to ignore the exit field when restoring the user
registers.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>