regulator: tps65217: Add device tree support

This commit adds device tree support for tps65217 pmic. And usage
details are added to device tree documentation. Driver is tested
by using kernel module with regulator set and get APIs.

Signed-off-by: AnilKumar Ch <anilkumar@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>