core: riscv: add initial RPMI/MPXY integration support

This commit introduces infrastructure to support RPMI communication via
the SBI MPXY extension. MPXY allows supervisor software to send and
rec

core: riscv: add initial RPMI/MPXY integration support

This commit introduces infrastructure to support RPMI communication via
the SBI MPXY extension. MPXY allows supervisor software to send and
receive RPMI messages through M-mode firmware acting as a proxy.

Functions added by this commit are sufficient to implement the
RequestForward service group.

Support for asynchronous events, notifications, and MSI-based message
delivery will be added in future commits.

Signed-off-by: Marouene Boubakri <marouene.boubakri@nxp.com>
Reviewed-by: Alvin Chang <alvinga@andestech.com>

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