mfd: Factor out WM831x I2C I/O from the core driver

In preparation for the addition of SPI support for the WM831x move the I2C
specific code into a separate file with a separate Kconfig option so the
I2C support can be excluded from the build.

Also update the 1133-EV1 PMIC module support for SMDK6410 to use the new
symbol.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>