mfd: Add VIA VX855 multi-function device support

This device has GPIO, SPI and I2C capabilities.

The hardware can be found in the OLPC XO-1.5 laptop.

Based on earlier work by Harald Welte.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>