lib: sbi: hart: Detect and enable Smrnmi before trap-based feature detection

The location of the RNMI/E trap vectors in the Smrnmi extension is
implementation-defined, so platforms with vendor-specific NMI vector
mechanisms must install the firmware's NMI entry points themselves.

Add an smrnmi_handlers_init() callback to sbi_platform_operations that
receives the firmware entry points and lets platform code install them
at the hardware-specific vector locations. Two pointers are passed:

  - _trap_rnmi_handler: the dedicated RNMI entry point that saves
    context using the Smrnmi MN* CSRs and returns via mnret.
  - _trap_handler: the regular M-mode trap entry since RNME is taken
    as a regular M-mode trap with NMIE=0.

When Smrnmi is present, install the platform's NMI vectors via the new
callback, initialize MNSCRATCH with the per-hart scratch pointer, and
set MNSTATUS.NMIE.

Smrnmi-enabled platforms must register smrnmi_handlers_init; if the
extension is detected but no callback is registered, sbi_panic() is
called since enabling NMIs without handlers in place would route
subsequent traps into nowhere.

Signed-off-by: Evgeny Voevodin <[email protected]>
Reviewed-by: Anup Patel <[email protected]>
Link: https://lore.kernel.org/r/88b1470e1e3348d454b4b995a11a85c01914f7af.1778176768.git.evvoevod@tenstorrent.com
Signed-off-by: Anup Patel <[email protected]>

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