arm: mvebu: PCIe support is now available on mvebu

Now that the PCIe driver for mvebu has been integrated and all its
relevant dependencies, we can mark the ARCH_MVEBU platform has
MIGHT_HAVE_PCI, which allows to select the PCI bus support if needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>