usb, davinci: add enable_vbus() weak function

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Remy Bohmer <linux@bohmer.net>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>