leds: lp55xx: support dynamic channel settings in the device tree structure

Currently, the LP55xx DT structure supports max 3 channels.
However, LP5523 has max 9 channels and LP5562 has 4 channels.
To enhance this constraint, the DT structure has been changed.

 (a) Use the child node for various channel settings instead of fixed array
 (b) Remove 'num_channel' property.
     This value can be retrieved by counting the children node.
 (c) 'chan-name' property supported
 (d) Documentation updates for LP5521 and LP5523

(cooloney@gmail.com: fix a coding style issue in leds-lp55xx.txt)

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