ath79: restore gpio-export on TL-WDR3600/4300

ath79: restore gpio-export on TL-WDR3600/4300

This partially reverts commit 32144ba275d163ce6e7d93546ab4414f03f508fb.

This commit replaced gpio-exports in favor of gpio-hogs for enabling USB
power at boot, but this rids the user of control of the USB port power
present on this device for a long time. It was agreed on a mailing list
[1] that this is not the way to go, and this patch breaks a very common
use-case of WWAN modem reset by power cycle, used on a lot USB equipped
routers, hence revert this change until a better solution can be found.

[1] http://lists.infradead.org/pipermail/openwrt-devel/2019-November/020151.html

Tested-by: Sungbo Eo <mans0n@gorani.run>
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
[adjusted commit title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>