leds: lp55xx: add common data structure for program

LP55xx family devices have internal three program engines which are used for
loading LED patterns. To maintain legacy device attributes, specific data
structure is used, 'mode' and 'led_mux'. The mode is used for showing/storing
current engine mode such like disabled, load and run. Then led_mux is used for
showing/storing current output LED selection.

This is only for LP5523/55231.

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