UNTESTED firmware: arm_sdei: Add support for manipulating shared event routing

The SDEI API provides a way to set the affinity of shared events. Add
API calls to allow the driver that registered the event to get and set
the routing using Linux's CPU numbers.

Any routing that is set is not saved or restored. In particular a
driver may route a shared event to a CPU that is offline, or shortly
go offline. Protecting against these are the callers responsibility.

Signed-off-by: James Morse <james.morse@arm.com>