bmips: shg2500: add missing led controller cells

bmips: shg2500: add missing led controller cells

Fixes the following DT warnings:
../dts/bcm63168-sercomm-shg2500.dts:76.4-14: Warning (reg_format): /ubus/spi@10001000/led-controller@1/led@1:reg: property has invalid length (4 bytes) (#address-cells == 2, #size-cells == 1)
../dts/bcm63168-sercomm-shg2500.dts:75.9-78.5: Warning (avoid_default_addr_size): /ubus/spi@10001000/led-controller@1/led@1: Relying on default #address-cells value
../dts/bcm63168-sercomm-shg2500.dts:75.9-78.5: Warning (avoid_default_addr_size): /ubus/spi@10001000/led-controller@1/led@1: Relying on default #size-cells value

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>