staging: mt7621-pci-phy: add new driver for phy part of mt7621-pci

Phy part of the pci for this SoC can be handled using a generic phy
driver. This commit extracts phy part of the mt7621-pci into a new
'mt7621-pci-phy' driver.

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