arm64: defconfig: enable xhci-platform

Some ARM64 platforms (for example the Marvell Armada 7K/8K) use the
generic XHCI platform driver, so it makes sense to enable
CONFIG_USB_XHCI_PLATFORM=y in the defconfig to support XHCI on such
platforms.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>