ARM64: dts: meson-gxbb: use the new GXBB DWMAC glue driver

The Amlogic reference driver uses the "mc_val" devicetree property to
configure the PRG_ETHERNET_ADDR0 register. Unfortunately it uses magic
values for this configuration.
According to the datasheet the PRG_ETHERNET_ADDR0 register is at address
0xc8834108. However, the reference driver uses 0xc8834540 instead.
According to my tests, the value from the reference driver is correct.

No changes are required to the board dts files because the only
required configuration option is the phy-mode, which had to be
configured correctly before as well.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>