staging: mt7621-pci: add reset control for general pcie reset

There is still a reset line which is not being handled using reset_control
properly and just being accessing writing registers. Use reset_control
instead for pcie general reset line.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>