[media] mn88473: convert to RegMap API

Convert driver to I2C RegMap API. That offers unified register
access routines, register value caching and more.

We need 3 register maps, one for each register page, as chips has
3 I2C address.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>