leds: tlc591xx: Document binding for the TI 8/16 Channel i2c LED driver

Document the binding for the TLC591xx LED driver.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Imre Kaloz <kaloz@openwrt.org>
Cc: Matthew.Fatheree@belkin.com
Acked-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>