I2C: mxc_i2c: get rid of __REG access

This driver accesses to processor's register
via __REG macros, that are removed (or are planned
to be removed) and replaced by C structures.
This patches replaces all occurrencies of __REG macros.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>