media: spi: Kconfig: Place SPI drivers on a single menu

It makes no sense to have two menus for SPI drivers, each
one with a single driver. Merge them and keep the Kconfig
sorted.

Suggested-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>