leds: lp5562: support the device tree feature

The LP55xx DT structure is applicable to the LP5562 device.
The driver and documentation are updated.

Compatible property of the DT
: LP5521 and LP5223 were manufactured by National Semiconductor.
  LP5562 is a new device from Texas Instruments.

Cc: Gabriel Fernandez <gabriel.fernandez@stericsson.com>
Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bryan Wu <cooloney@gmail.com>