MFD: TPS65910: Add new mfd device for TPS65910

The TPS65910 chip is a power management IC for multimedia and handheld
devices. It contains the following components:

- Regulators
- GPIO controller
- RTC

The tps65910 core driver is registered as a platform driver and provides
communication through I2C with the host device for the different
components.

Signed-off-by: Graeme Gregory <gg@slimlogic.co.uk>
Signed-off-by: Jorge Eduardo Candelaria <jedu@slimlogic.co.uk>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>