arm64: dts: meson-g12a-sei510: Enable Wifi SDIO module

The SEI510 embeds an AP6398S SDIO module, let's add the
corresponding SDIO, PWM clock and mmc-pwrseq nodes.

Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>