ARM: dts: meson: provide the XTAL clock using a fixed-clock

The clock controller driver has provided the XTAL clock so far. This
does not match how the hardware actually works because the XTAL clock is
an actual crystal which is mounted on the PCB.

Add the "xtal" clock to meson.dtsi and replace all references to the
clock controller's CLKID_XTAL with the new xtal clock node.

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