PCI: mt7621: Allow COMPILE_TEST for all arches

Since all MIPS-specific code has been removed from driver, allow it to be
enabled for COMPILE_TEST on all architectures.

Mark it as tristate and remove MIPS the MIPS dependency.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>