leds: Add new LED driver for lm355x chips
LM3554 and LM3556 have similar functions but very different register map.
This driver is a general version for lm3554 and lm3556 both,led chips of TI.
lm3556 driver can be replaced by this driver.

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

https://patchwork.kernel.org/patch/1398791/

Change-Id: I49fcd18c9930cd2d0158754487355e41bc602f19