MFD: TPS65910: Add support for TPS65911 device

The TPS65911 is the next generation of the TPS65910 family of
PMIC chips. It adds a few features:

- Watchdog Timer
- PWM & LED generators
- Comparators for system control status

It also adds a set of Interrupts and GPIOs, among other things.

The driver exports a function to identify between different
versions of the tps65910 family, allowing other modules to
identify the capabilities of the current chip.

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>