ARM: tegra: don't hard-code USB ULPI PHY reset_gpio

Not all boards use GPIO_PV0 as the ULPI PHY reset signal. Instead of
hard-coding this GPIO into devices.c, make the board files set it
explicitly. This will allow the PHY code to differentiate between set and
unset values, and hence know when to read the value from device tree.

Signed-off-by: Stephen Warren <swarren@nvidia.com>