USB: ehci-s5p: Add vbus setup function to the s5p ehci glue layer

This patch retrieves and configures the vbus control gpio via
the device tree. The suspend/resume callbacks will be later
modified for vbus control.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>