riscv: sbi: Switch to new sys-off handler API

riscv: sbi: Switch to new sys-off handler API

Kernel now supports chained power-off handlers. Use
register_platform_power_off() that registers a platform level power-off
handler. Legacy pm_power_off() will be removed once all drivers and archs
are converted to the new sys-off API.

Signed-off-by: Andrew Davis <afd@ti.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250813151855.105237-1-afd@ti.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>