at91: ohci-at91: add vbus_pin_inverted platform attribute

The existing OHCI AT91 driver made the assumption that the enable
input of the USB power switch was active low. However, some USB power
switches such as the Micrel MIC2026-1 [1] have an active high input to
enable the power. A new vbus_pin_inverted attribute is added to the
at91_usbh_data structure so that board files can tell the OHCI driver
if the vbus pin logic is active low or active high.

[1] http://www.micrel.com/page.do?page=product-info/products/mic2026.shtml

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>