ARM: dts: meson: add the clock inputs for the Meson timer

The Meson Timer IP block has two clock inputs:
- clk81 for using the system clock as timebase
- xtal for a timebase with 1us, 10us, 100us and 1ms resolution

The clocksource driver does not use these yet, but it's still a good
idea to add them as this describes how the hardware actually works
internally.

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