ramips: allow USB power control on TP-Link MR3020v3

ramips: allow USB power control on TP-Link MR3020v3

By switching EPHY_LED4_N_JTRST_N from EPHY_LED4_N to GPIO#39
we can control USB port power an all current revisions of MR3020v3.
It was not a thing on some first revisions, pin was unused.
But for now on all current MR3020v3 boards EPHY_LED4_N_JTRST_N pin
is connected to USB power key.
Also it was not used as EPHY indicator on any revision of the board.

Signed-off-by: Dmitry Chigiryov <dmitry.chigiryov@ya.ru>
[changed author address (used SoB one)]
Signed-off-by: Sungbo Eo <mans0n@gorani.run>