ARM: dts: meson: Fix the UART compatible strings

The dt-bindings for the UART controller only allow the following values
for Meson6 SoCs:
- "amlogic,meson6-uart", "amlogic,meson-ao-uart"
- "amlogic,meson6-uart"

Use the correct fallback compatible string "amlogic,meson-ao-uart" for
AO UART. Drop the "amlogic,meson-uart" compatible string from the EE
domain UART controllers.

Fixes: ec9b5916

 ("ARM: dts: meson6: use stable UART bindings")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20211227180026.4068352-2-martin.blumenstingl@googlemail.com