Add TF-M Core API for requests by Secure Partitions to SPM

SVC calls can be used between a Trusted (PSA RoT) service and TF-M Core for interaction, that interface is implementation defined.

A dedicated SVC number (TFM_SVC_SPM_REQUEST) is called with the first parameter (i.e. r0 at the point of SVC call) identifying the type of request (e.g. reset request vote, low power mode vote, etc), r1-r3 carrying type-specific additional parameters.
r0 on return from exception contains result.
If the SVC call is implemented in a naked ASM function, the caller C function can handle input parameters and function return value according to AAPCS.

Initial implementation contains a single request type, used by platform service to request an indication from SPM whether a system reset request can be permitted at a given point in time or not.