[media] v4l: omap4iss: Replace outdated OMAP4 control pad API with syscon

The omap4_ctrl_pad_readl and omap4_ctrl_pad_writel functions have been
removed by commit efde2346 but are still used by the OMAP4 ISS
driver, resulting in a compilation breakage:

drivers/staging/media/omap4iss/iss_csiphy.c: In function 'omap4iss_csiphy_config':
drivers/staging/media/omap4iss/iss_csiphy.c:167:2: error: implicit declaration of function 'omap4_ctrl_pad_writel' [-Werror=implicit-function-declaration]
  omap4_ctrl_pad_writel(cam_rx_ctrl,

Fix the problem by using the syscon API to reaplace the control pad API.
Lookup the syscon instance by compatible name for now as the OMAP4 ISS
driver doesn't support DT yet.

Fixes: efde2346

 ("ARM: OMAP4+: control: remove support for legacy pad read/write")

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Alius <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>