dt-bindings: timer: meson6_timer: document all interrupts

The meson6_timer IP block supports four timers - each of them has it's
own interrupt line. Update the documentation to reflect that all four
interrupts should be passed.

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