mips: ath79: dts: Add generic-ehci node

Add generic EHCI node for the ChipIdea EHCI controller in the ath79.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Wills Wang <wills.wang@live.com>