media: rcar-csi2: Move PHTW write helpers

media: rcar-csi2: Move PHTW write helpers

Prepare for V4M support by moving the PHTW write helpers to the generic
write helpers. This is needed as adding V4M support will involve
interact with the PHTW register from code that are logically grouped
with similar code in such a way that forward declarations of these
helpers would otherwise be needed.

The functions are moved verbatim.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>