Documentation/bindings: net: marvell-pp2: update the IRQs description

This patch updates the interrupts part of the Marvell PPv2 driver
bindings documentation, to keep it in sync with the driver.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>