bindings: net: stmmac: add missing note about LPI interrupt

The hardware has a LPI interrupt.
There is already code in the stmmac driver to parse and handle the
interrupt. However, this information was missing from the DT binding.

At the same time, improve the description of the existing interrupts.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Acked-By: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>