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>

arch/arm64/configs/defconfig

@@ -256,6 +256,7 @@ CONFIG_SND_SOC_AK4613=y
 CONFIG_USB=y
 CONFIG_USB_OTG=y
 CONFIG_USB_XHCI_HCD=y
+CONFIG_USB_XHCI_PLATFORM=y
 CONFIG_USB_XHCI_RCAR=y
 CONFIG_USB_EHCI_EXYNOS=y
 CONFIG_USB_XHCI_TEGRA=y