firmware: arm_ffa: Initial support for scheduler receiver interrupt

firmware: arm_ffa: Initial support for scheduler receiver interrupt

The Framework uses the schedule receiver interrupt to inform the
receiver’s scheduler that the receiver must be run to handle a pending
notification. A receiver’s scheduler can obtain the description of the
schedule receiver interrupt by invoking the FFA_FEATURES interface.

The delivery of the physical schedule receiver interrupt from the secure
state to the non-secure state depends upon the state of the interrupt
controller as configured by the hypervisor.

The schedule seceiver interrupt is assumed to be a SGI. The Arm GIC
specification defines 16 SGIs. It recommends that they are equally
divided between the non-secure and secure states. OS like Linux kernel
in the non-secure state typically do not have SGIs to spare. The usage
of SGIs in the secure state is however limited. It is more likely that
software in the Secure world does not use all the SGIs allocated to it.

It is recommended that the secure world software donates an unused SGI
to the normal world for use as the schedule receiver interrupt. This
implies that secure world software must configure the SGI in the GIC
as a non-secure interrupt before presenting it to the normal world.

Link: https://lore.kernel.org/r/20231005-ffa_v1-1_notif-v4-8-cddd3237809c@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>