sunxi: Fix USB regulators in Linksprite_pcDuino_defconfig

The pcDuino1 board unconditionally provides 5V to USB host
receptacles. The pcDuino2 board has a voltage regulator,
controlled by the PD2 pin which is pulled-up by default
(so that the USB power is also enabled by default).

Not specifying pins for enabling USB power in the defconfig
means that the PH3 and PH6 pins are driven high by default.
The PH6 pin is available on the Arduino-compatible expansion
header and touching it is not nice (this may be even dangerous,
depending on what kind of role is assigned to this particular
pin by various Arduino shields).

This patch explicitly configures the USB VBUS pins to "",
which means that no pins should be touched. The patch has
been tested on a pcDuino2 board and USB still works.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>