arm64: defconfig: set CONFIG_USB_CONN_GPIO=y

Since 10a7b37b ("arm64: Update default configuration") PHY_TEGRA_XUSB
is set to y. That option needs USB_CONN_GPIO to also be y not m, so
set USB_CONN_GPIO explicitly to y to reflect reality.

Signed-off-by: Ross Burton <ross.burton@arm.com>