ARM: dts: meson8: add the xtal_32k_out pin

GPIOX_10 can generate a 32768Hz signal when enabling the "xtal_32k_out"
group with the xtal function. This is typically used as LPO clock for
the SDIO wifi chips.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>