leds: Add new LED driver for lm3642 chips

This driver is a general version for LM642 led chip of TI.

LM3642 :
The LM3642 is a 4MHz fixed-frequency synchronous boost
converter plus 1.5A constant current driver for a high-current
white LED.
The LM3642 is controlled via an I2C-compatible interface.

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