arm64: defconfig: add Allwinner USB PHY

The arm64 defconfig covers the Allwinner A64 SoC boards quite well,
but USB support is not enabled.
Add the PHY config symbol to allow defconfig kernels to use USB.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>