[media] v4l: omap4iss: Create and use register access functions

Replace the direct readl/writel calls with helper functions that take an
ISS pointer and compute the register memory address. Also add bit clear,
set and update helpers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>