leds: add LED driver for lm3556 chip

leds: add LED driver for lm3556 chip

A simple driver for the Texas Instruments LM3556 chip.

The LM3556 is a 4 MHz fixed-frequency synchronous boost converter plus
1.5A constant current driver for a high-current white LED.  Datasheet:
www.national.com/ds/LM/LM3556.pdf

Tested on OMAP4430

Signed-off-by: Geon Si Jeong <gshark.jeong@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Daniel Jeong <daniel.jeong@ti.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Greg KH <greg@kroah.com>
Cc: Wolfram Sang <w.sang@pengutronix.de>
Cc: Shuah Khan <shuahkhan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>