ramips: fix ethernet MAC address on Omega2

According to https://docs.onion.io/omega2-docs/mac-address.html, 0x28 is
the correct location to read the address on Onion Omega 2(+) devices.

This fixes a regression introduced by commit 77e850fe76e3 ("ramips: tidy up
MAC address setup for Linkit Smart and Omega2"), which was a cleanup that
intended to preserve existing behavior. In my testing with v19.07.7,
however, the MAC address determined from the device tree takes precedence
over the one set by 02_network, so the aforementioned commit actually
changed the behavior.

Signed-off-by: Michael Siegenthaler <[email protected]>
[Adapt patch to nvmem usage]
Signed-off-by: Hauke Mehrtens <[email protected]>

target/linux/ramips/dts/mt7628an_onion_omega2.dtsi

@@ -152,7 +152,7 @@
 };
 
 &ethernet {
-	nvmem-cells = <&macaddr_factory_2e>;
+	nvmem-cells = <&macaddr_factory_28>;
 	nvmem-cell-names = "mac-address";
 };
 
@@ -172,7 +172,7 @@
 	#address-cells = <1>;
 	#size-cells = <1>;
 
-	macaddr_factory_2e: macaddr@2e {
-		reg = <0x2e 0x6>;
+	macaddr_factory_28: macaddr@28 {
+		reg = <0x28 0x6>;
 	};
 };