MIPS: dts: ralink: gardena_smart_gateway_mt7688: Fix power LED

MIPS: dts: ralink: gardena_smart_gateway_mt7688: Fix power LED

When starting up, the GARDENA smart Gateway's power LED should be
flashing green. It is unclear why it was initially set to "off".

The LED frequency cannot be configured in the devicetree. Luckily, the
default is 1 Hz, which is what we want.

Signed-off-by: Ezra Buehler <ezra.buehler@husqvarnagroup.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>