spi: Document cs_gpios and cs_gpio in kernel-doc

This adds missing kernel-doc entries for cs_gpios in struct spi_master and
cs_gpio in struct spi_device.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
[grant.likely: tweaked the language of the descriptions]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>