musb: Add Phy programming for using external Vbus

MUSB PHY on OMAP3EVM Rev >= E uses external Vbus supply to support
500mA of power.We need to program MUSB PHY to use external Vbus
for this purpose.

Adding 'extvbus' member in musb_config structure which should be set
by all the boards where MUSB interface is using external Vbus supply.

Also added ULPI bus control register read/write abstraction for
Blackfin processor as it doesn't have ULPI registers.

CC: Remy Bohmer <linux@bohmer.net>
Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>