ramips: RB750Gr3: Add pwr LED and buzzer to DTS

use pwr LED in diag.sh
Expose unused pinmux pins as GPIOs
export power LED and buzzer pins
Use rb750gr3:blue:pwr LED in diag.sh for boot status instead of rb750gr3:green:usr

Signed-off-by: Andrew Yong <me@ndoo.sg>