asm/arch-meson: add missing meson_sm_write_efuse signature

This function can be used by other modules. Also add comments.

Signed-off-by: Alexey Romanov <avromanov@sberdevices.ru>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20230110105650.54580-2-avromanov@sberdevices.ru


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>