ath79: ar934x: use reset for usb-phy-analog

ath79: ar934x: use reset for usb-phy-analog

This was already available on ar71xx, but is missing on ath79.
This solves the slow usb speed on TP-Link WDR3600/WDR4300 and similar,
as reported in Flyspray [0], OpenWRT Forum [1] and GitHub PR [2].

[0] https://bugs.openwrt.org/index.php?do=details&task_id=2567
[1] https://forum.openwrt.org/t/usb-wdr4300-low-speed-on-external-storage/46794
[2] https://github.com/openwrt/openwrt/pull/964

Tested-By: Lech Perczak <lech.perczak@gmail.com> [TL-WDR4300]
Signed-off-by: Johann Neuhauser <johann@it-neuhauser.de>
(cherry picked from commit bda6b6144dbe3e12d128b500821799ef472de4cb)