media: sun6i-mipi-csi2: Depend on PHY_SUN6I_MIPI_DPHY

PHY_SUN6I_MIPI_DPHY is not a freely selectable option and so may not
always be available. Depend on it instead.

Fixes: 94d7fd96 ("media: sunxi: Depend on GENERIC_PHY_MIPI_DPHY")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>