dts: mt7629: enable JTAG pins by default

The EPHY LEDs belongs to the built-in FE switch of MT7629, which is barely
used. These LED pins on reference boards are used as JTAG socket. So it's
a good idea to change the default state to JTAG, and this will make it
convenience for debugging.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>