ARM: dts: meson8b: ec100: wire up the RT5640 audio codec

The Realtek RT5640 codec is connected to the SoC's I2S interface.
Describe this in the .dts together with the codec's LDO1 enable GPIO so
audio can be played on the Endless Mini.
While here, add a note about the realtek,ldo1-en-gpios for which the
EC100 uses GPIO_BSD_EN. Due to driver limitations this pin cannot be
used currently.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20210717233030.331273-3-martin.blumenstingl@googlemail.com