PCI: mt7621: Add missing MODULE_LICENSE()

PCI: mt7621: Add missing MODULE_LICENSE()

The MT7621 PCIe host controller driver can be built as a module, but it
lacks a MODULE_LICENSE(), which causes a build error:

  ERROR: modpost: missing MODULE_LICENSE() in drivers/pci/controller/pcie-mt7621.o

Add MODULE_LICENSE() to the driver.

Fixes: 2bdd5238e756 ("PCI: mt7621: Add MediaTek MT7621 PCIe host controller driver")
Link: https://lore.kernel.org/r/20211207104924.21327-5-sergio.paracuellos@gmail.com
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Krzysztof Wilczyński <kw@linux.com>