media: v4l2-core: move spi helpers out of v4l2-common.c

Separate the spi helpers to v4l2-spi.c, in order to get rid
of the ifdefery. No functional changes intended, this is
just a cosmetic change to organize the code better.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>