mfd: Move the mc13xxx-core spi specific code into a separate module

All spi specific code is moved into a new module. The mc13xxx struct
moves to a new local include file by necessity.

A new config choice selects the SPI bus type support and by default is
value of SPI_MASTER to remain compatible with existing configs.

Signed-off-by: Marc Reilly <marc@cpdesign.com.au>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>