ramips: simplify status for ehci/ohci on mt7628 TP-Link devices

ramips: simplify status for ehci/ohci on mt7628 TP-Link devices

At the moment, ehci/ohci is enabled in mt7628an SoC DTSI, then
disabled in the TP-Link-specific DTSI files, and finally enabled
again in the DTS files of the devices needing it.

This on-off-on scheme is hard to grasp on a quick look. Thus, this
patch drops the status in the TP-Link-specific DTSI files, having
the TP-Link devices treated like the rest of mt7628an DTSes, i.e.
ehci/ohci is enabled by default and needs to be disabled explicitly
where needed.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>