Add driver for the PCIe controller of the MT7621 SoC.

[bhelgaas: rename from pci-mt7621.c to pcie-mt7621.c; also rename Kconfig
symbol from PCI_MT7621 to PCIE_MT7621]
Link: https://lore.kernel.org/r/20210922050035.18162-3-sergio.paracuellos@gmail.comSigned-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>