arm64: dts: meson: remove extra subnode in mmc clk_gate pinmux

In the pinmux of the mmc clk_gate nodes, we define 2 subnodes. One for
the function definition, the other for the bias. This is not necessary
since we can define the function and the bias in the same subnode.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>