core: riscv: implement MPXY SBI extension

Introduce support for the MPXY SBI extension (EID #0x4D505859),
which enables supervisor software to exchange messages through
an abstract SBI interface dec

core: riscv: implement MPXY SBI extension

Introduce support for the MPXY SBI extension (EID #0x4D505859),
which enables supervisor software to exchange messages through
an abstract SBI interface decoupled from any specific protocol.

MPXY allows setting up per-hart shared memory for transferring
messages between the supervisor and an SBI implementation.
It serves as a generic backend to build messaging clients,
while being agnostic of message protocol semantics.

This step provides the foundation for future inter-domain
message exchange.

Signed-off-by: Marouene Boubakri <marouene.boubakri@nxp.com>
Reviewed-by: Yu-Chien Peter Lin <peter.lin@sifive.com>
Reviewed-by: Alvin Chang <alvinga@andestech.com>

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