media: staging: rkisp1 Kconfig: depends on OF

building it with a random config causes a warning:

WARNING: unmet direct dependencies detected for PHY_ROCKCHIP_DPHY_RX0
  Depends on [n]: STAGING [=y] && STAGING_MEDIA [=y] && MEDIA_SUPPORT [=y] && (ARCH_ROCKCHIP || COMPILE_TEST [=y]) && OF [=n]
  Selected by [y]:
  - VIDEO_ROCKCHIP_ISP1 [=y] && STAGING [=y] && STAGING_MEDIA [=y] && MEDIA_SUPPORT [=y] && VIDEO_V4L2 [=y] && (ARCH_ROCKCHIP || COMPILE_TEST [=y])

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>