ARM: dts: meson8b: extend ethernet controller description

Enable S805 (aka Meson8b) ethernet pin multiplexing and
extend the controller description.
The programmable ethernet (PRG_ETHERNET) register address
value (0xc1108108), contained in meson.dtsi, is overridden
according to the value found in S805 SoC manual.
This also required to switch to "amlogic,meson8b-dwmac" compatible
to correctly configure that register.
The two clock sources "clkin0" and "clkin1" are both equals
to MPLL2 because, as reported in bit 9-7 register description,
that is the only Meson8b ethernet clock source.

Signed-off-by: Emiliano Ingrassia <ingrassia@epigenesys.com>
Tested-by: Linus Lüssing <linus.luessing@c0d3.blue>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>