mips: dts: align LED node names with dtschema

The node names should be generic and DT schema expects certain pattern:

  mt7621-gnubee-gb-pc1.dtb: gpio-leds: 'power', 'system' do not match any of the regexes: '(^led-[0-9a-f]$|led)', 'pinctrl-[0-9]+'

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>