ARM: dt: tegra trimslice: add vbus-gpio property

On TrimSlice, Tegra's USB1 port may be routed to either an external micro
USB port, or an internal USB->SATA bridge for SSD or HDD. This muxing is
controlled by a GPIO. Whilst not strictly a VBUS GPIO, the TrimSlice
board files caused this GPIO to be set appropriately to enable the SATA
bridge by passing it as the VBUS GPIO to the USB driver. Echo this same
configuration in device tree to enable the SATA bridge.

An alternative might be to implement a full USB bus mux driver. However,
that seems over-complex right now.

Cc: <stable@vger.kernel.org> # v3.5 (file is named tegra-trimslice.dts there)
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>