media: imx: imx7_mipi_csis: Move PHY control to dedicated functions

Move the PHY regulator and reset handling to dedicated functions. This
groups all related code together, and prepares for i.MX8 support that
doesn't require control of the PHY regulator and reset.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>