spi: Add APIs in spi core to set/get spi->chip_select and spi->cs_gpiod

spi: Add APIs in spi core to set/get spi->chip_select and spi->cs_gpiod

Supporting multi-cs in spi core and spi controller drivers would require
the chip_select & cs_gpiod members of struct spi_device to be an array.
But changing the type of these members to array would break the spi driver
functionality. To make the transition smoother introduced four new APIs to
get/set the spi->chip_select & spi->cs_gpiod and replaced all
spi->chip_select and spi->cs_gpiod references in spi core with the API
calls.
While adding multi-cs support in further patches the chip_select & cs_gpiod
members of the spi_device structure would be converted to arrays & the
"idx" parameter of the APIs would be used as array index i.e.,
spi->chip_select[idx] & spi->cs_gpiod[idx] respectively.

Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@amd.com>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20230119185342.2093323-2-amit.kumar-mahapatra@amd.com
Signed-off-by: Mark Brown <broonie@kernel.org>