o4-imx-nano: Fix dtc warnings

Remove the LED unit addresses and reg properties to
fix the following dtc build warnings:

arch/arm/dts/o4-imx-nano.dtb: Warning (reg_format): /leds/led@0:reg: property has invalid length (4 bytes) (#address-cells == 2, #size-cells == 1)
...
arch/arm/dts/o4-imx-nano.dtb: Warning (avoid_default_addr_size): /leds/led@0: Relying on default #address-cells value

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Oleh Kravchenko <oleg@kaa.org.ua>