i2c: mv64xxx: Add macros to access parts of registers

These macros make it more comprehensive to access to useful masked and
shifted area of the various registers used.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>