leds: Add new LED driver for lm355x chips

This driver is a general version for LM355x,lm3554 and lm3556,led chips of TI.

LM3554 :
The LM3554 is a 2 MHz fixed-frequency synchronous boost
converter with 1.2A dual high side led drivers.
Datasheet: www.ti.com/lit/ds/symlink/lm3554.pdf

LM3556 :
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

(bryan.wu@canonical.com: use flush_work() to replace flush_work_sync() which is
deprecated)

Signed-off-by: G.Shark Jeong <gshark.jeong@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>