drivers: i2c: mentor: move platform code into header files

At the moment we have two I2C stub drivers (for the Allwinner and the
Marvell platform), which #include the actual .c driver file.
Change this into the more usual design, by renaming and moving the stub
drivers into platform specific header files and including these from the
actual driver file. The platform specific include directories make sure
the driver picks up the right header automatically.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>