leds-lp55xx: use lp55xx_init_led() common function

 lp5521_init_led() and lp5523_init_led() are replaced with one common function,
 lp55xx_init_led().
 Max channels is configurable, so it's used in lp55xx_init_led().

 'LP5523_LEDS' are changed to 'LP5523_MAX_LEDS'.

 lp55xx_set_brightness, lp55xx_led_attributes: skeleton
 Will be filled in next patches.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>