[I2C] Split mv643xx I2C platform support

The motivation for this change is to allow other chips, like the
Marvell Orion ARM SoC family, to use the existing i2c-mv64xxx driver.

Signed-off-by: Tzachi Perelstein <tzachi@marvell.com>
Acked-by: Nicolas Pitre <nico@marvell.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>
Acked-by: Mark A. Greer <mgreer@mvista.com>
Acked-by: Jean Delvare <khali@linux-fr.org>