[media] omap3isp: Replace mmio_base_phys array with the histogram block base

Only the histogram sub-block driver uses the physical address. Do not store
it for other sub-blocks.

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