samples: drivers: add test sample for TI LP503x LED drivers

This sample controls up to 12 LEDs connected to a LP503x driver.

First, for each LED information is retrieved using the led_get_info
syscall and printed in the log messages. Next, from an infinite loop, a
test pattern (described below) is applied to all the LEDs simultaneously
(using the led_write_channels syscall) and then to each LED one by one
(using the led_set_{brightness,color} syscalls).

Test pattern:

 For each color in red green blue white yellow purple cyan orange:
  - set the color
  - turn on
  - turn off
  - set the brightness gradually to the maximum level
  - turn off

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