dt-binding: net/phy: fix interrupts description

Commit b053dc5a (powerpc: Refactor device tree binding) split the
Ethernet PHY binding documentation out of the big booting-without-of.txt
file, leaving a dangling reference to "section 2" in the 'interrupts'
property description. Drop that reference, and make the description look
more like the rest.

While at it, make the example interrupt-parent phandle look more like a
real world phandle, and use an IRQ_TYPE_ macro for the 'interrupts'
type.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Rob Herring <robh@kernel.org>