ARM: davinci: da830-evm: add a fixed regulator for ohci-da8xx

Instead of directly using the vbus GPIO we should model it as a fixed
regulator. Add all necessary fix-ups for the regulator to be registered
and configure the vbus GPIO as its enable pin.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>