firmware: Add RNMI handler infrastructure

Implement basic Resumable NMI (RNMI) handler support for the RISC-V
Smrnmi extension.

The new _trap_rnmi_handler assembly entry point saves context using the
Smrnmi MN* CSRs (MNSCRATCH, MNEPC, MNSTATUS, MNCAUSE) and returns via
mnret. It dispatches to sbi_trap_rnmi_handler(), which optionally calls
a platform-specific ops->rnmi_handler callback for actual NMI
processing. If no platform handler is registered or it fails, the
event is reported as an unhandled NMI.

The RNMI handler reuses the generic trap context structure but stores MN*
CSR values (MNEPC, MNSTATUS, MNCAUSE) into the corresponding generic
fields (mepc, mstatus, cause) for compatibility with existing trap
infrastructure.

Signed-off-by: Evgeny Voevodin <[email protected]>
Reviewed-by: Anup Patel <[email protected]>
Link: https://lore.kernel.org/r/050ae6d2762ba8d5b9dfb3cc1960a23aa3d6c549.1778176768.git.evvoevod@tenstorrent.com
Signed-off-by: Anup Patel <[email protected]>

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