This commit:
1. Removes deprecated "label" property from the dts leds subnnodes;
2. Updates buttons and leds dts description according to kernel docs
examples.
Scope: devices well known to me.
Run-tested: TP-Link ec330-g5u, WiFire S1500.nbn
Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>