net: mediatek: update the IRQ part of the binding document

The current binding document only describes a single interrupt. Update the
document by adding the 2 other interrupts.

The driver currently only uses a single interrupt. The HW is however able
to using IRQ grouping to split TX and RX onto separate GIC irqs.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>