led: added cmd_led to Makefile

Addition of cmd_led into the Makefile wasn't included in the patch
applied to u-boot-ti.

Signed-off-by: Jason Kridner <jkridner@beagleboard.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Joel A Fernandes <agnel.joel@gmail.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>