firmware: arm_ffa: Refactor SRI handling in prepartion to add NPI support

firmware: arm_ffa: Refactor SRI handling in prepartion to add NPI support

In preparation to support handling of Notification Pending Interrupt(NPI)
in addition to the existing support for Schedule Receiver Interrupt(SRI),
refactor the code around SRI handling so that NPI support can reuse some
of it. This change shouldn't have any functionality impact. It neither
adds the support for NPIs nor changes any SRI support.

Tested-by: Jens Wiklander <jens.wiklander@linaro.org>
Link: https://lore.kernel.org/r/20240411-ffa_npi_support-v2-2-927a670254e6@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>