serial: meson: Update compatible with new Linux bindings
The Amlogic Meson SoCs serial bindings were not written when serial
support was pushed into Linux and U-Boot.
A clean bindings document has been merged into Linux tree to correctly
handle the multiple clocks feeding the serial peripheral.
This update the U-Boot serial_meson driver with the new compatible
string for Amlogic Meson GX Socs.
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com> Reviewed-by: Simon Glass <sjg@chromium.org>