ARM: imx_v6_v7_defconfig: enable PCI_MSI

This has been stable on i.MX6 for a good while now and there is no
reason to keep it disabled, as it allows to allocate non-shared
IRQs if several PCIe devices are connected to a single host.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>