leds-lp55xx: add new lp55xx_register_sysfs() for the firmware interface

 LP55xx family chips have internal program memory which run various patterns.
 Using this memory, LEDs continue on blinking/dimming without continuous I2C
 commands. That means the I2C HOST can be entered into sleep once the memory
 is updated.

 An application can get hex data from a file and write them into
 the program memory through the I2C. This is general firwmare interface.

 This patch is the initial step for adding the firmware interface.

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