ARM: dts: sun8i: r40: Add device node and RGMII pinmux node for GMAC

The R40 SoC has a GMAC (gigabit capable Ethernet controller). Add a
device node for it. The only publicly available board for this SoC
uses an RGMII PHY. Add a pinmux node for it as well.

Since this SoC also has an old 10/100 Mbps EMAC, which also has an
MDIO bus controller, the MDIO bus for the GMAC is labeled "gmac_mdio".

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>