ramips: mt7621: enable unused PCIe port to fix potential boot failure

ramips: mt7621: enable unused PCIe port to fix potential boot failure

One user reported that his SIMAX1800T couldn't boot like the others. After
debugging, I found that this was caused by the disabled PCIe port. I cannot
reproduce this issue on my SIMAX1800T. But when I disabled pcie2 on the
ASUS RT-AC57U, I got the same result.

It seems that disabling these unused PCIe ports on some mt7621 revisions
will cause PCIe to fail to initialize. So we'd better to re-enable them on
all related mt7621 devices.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>