mfd: twl-core: Add initial DT support for twl4030/twl6030

Add initial device-tree support for twl familly chips.
The current version is missing the regulator entries due
to the lack of DT regulator bindings for the moment.
Only the simple sub-modules that do not depend on
platform_data information can be initialized properly.

Add irqdomain support.

Add documentation for the Texas Instruments TWL Integrated Chip.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Balaji T K <balajitk@ti.com>
Cc: Graeme Gregory <gg@slimlogic.co.uk>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
[grant.likely@secretlab.ca: Fix IRQ_DOMAIN dependency in kconfig]
Cc: Grant Likely <grant.likely@secretlab.ca>