media: ti-vpe: cal: Split CAMERARX syscon regmap retrieval to a function

Simplify the cal_probe() function by splitting the CAMERARX syscon
regmap retrieval to a separate function. A few local variables are
renamed in the process to shorten them (syscon_camerrx_*) or to make
them more accurate (parent isn't the parent OF node but the CAL device's
own OF node).

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>