ipq807x: add nvmem uboot-env support

ipq807x: add nvmem uboot-env support

Enabling kernel symbol CONFIG_NVMEM_U_BOOT_ENV allows to use u-boot
environement variable ethaddr with nvmen. That way it is possible to assign
the MAC address to the ethernet device driver.

Example of usage in dts:
....
	partition@600000 {
			compatible = "u-boot,env";
			label = "0:appsblenv";
			reg = <0x600000 0x10000>;

			macaddr_lan: ethaddr {
			};
	};
....
&dp5 {
	status = "okay";
	phy-handle = <&qca8081>;
	label = "wan";
	nvmem-cells = <&macaddr_lan>;
	nvmem-cell-names = "mac-address-ascii";
	mac-address-increment = <1>;
};

This is needed for Zyxel NBG7815.

Signed-off-by: André Valentin <avalentin@marcant.net>