mfd: Prepare for separating spi and i2c mc13xxx-core backends

This patch abstracts the bus specific operations from the driver core.
Generic init and cleanup is consolidated into mc13xxx_common_*.
spi specific functions are renamed to reflect such.
(The irq member of the mc13xxx struct is no longer redundant, it's used
to store the irq for cleanup time).

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