spi: amd: Add struct and enum kernel-doc comments

Provide documentation comments in the kernel-doc format
for enum amd_spi_versions and struct amd_spi.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20220706100626.1234731-6-cristian.ciocaltea@collabora.com


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