pinctrl: meson: meson8: add the RGMII RXD2/RXD3 and TXD2/TXD3 signals

These are only available on the Meson8m2 SoC (which uses the same
DesignWare Ethernet MAC as Meson8b).
The "eth_tx_clk_50m" signal either provides a 50MHz clock for the RMII
PHYs or the RGMII TX clock (as far as we know the frequency is
controlled by the PRG_ETHERNET registers in the Ethernet MAC "glue" IP
block).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>