backlight: Add TPS65217 WLED driver

The TPS65217 chip contains a boost converter and current sinks which can be
used to drive LEDs for use as backlights. Expose this functionality via the
backlight API.

Tested on an AM335x based custom board with a single WLED string, using
different values for ISEL and FDIM (though it would be hard to tell the
difference except for the value in WLEDCTRL1). Both instantiation through the
device tree and by passing platform data have been tested. Testing has been
done with an Androidized 3.2 kernel from the rowboat project. Koen Kooi
reported the driver to be working on a Beaglebone board with LCD3 cape

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>