spi: Doc fix - Describe add_lock and dma_map_dev in spi_controller

This fixes the corresponding warnings during building the docs.

Signed-off-by: Siddh Raman Pant <siddhpant.gh@gmail.com>
Link: https://lore.kernel.org/r/4e6187a4-d0f8-4750-e407-e09cc1c91789@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>