mfd: tps65218: Add driver for the TPS65218 PMIC

The TPS65218 chip is a power management IC for Portable Navigation Systems
and Tablet Computing devices. It contains the following components:

 - Regulators.
 - Over Temperature warning and Shut down.

This patch adds support for tps65218 mfd device. At this time only
the regulator functionality is made available.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>