drivers: led: add driver for TI LP503x controllers

This patch adds support for the Texas Instruments LP5030 and LP5036
I2C LED controllers. They are respectively providing up to 30 and 36
channels (i.e. 10 or 12 RGB LEDs).

In addition to the channel/color registers this LED controller provides
a per-LED brigthness register. This driver implements both LED-based and
channel-based API methods:

- led_on
- led_off
- led_get_info
- led_set_brightness
- led_set_color
- led_set_channel
- led_write_channels

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>