arm: dispatch hypercalls

arm: dispatch hypercalls

Initial code for handling hypervisor traps. Only the non-banked registers
need to be saved in the low-level handler, the rest of the context won't
be overwritten.
The per-cpu datas are loaded from TPIDR_EL2 and the general-purpose
registers are saved directly on the stack and supplied as 'struct
registers' to the dispatcher.
The latter then inspects the ESR value and calls the core accordingly.
The return value and the general-purpose registers are passed back to the
driver by retrieving 'struct registers' from the stack, before doing the
final ERET.
This patch allows to handle all status querying hypercalls.

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