PCI: brcmstb: Advertise MSI-X support

Although the BRCMSTB PCIe interface doesn't technically support the
MSI-X spec, in practise it seems to work provided no more than 32
MSI-Xs are required. Add the required flag to the driver to allow
experimentation with devices that demand MSI-X support.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>