OMAP3+: Clock: Adding ehci clock enabling

OMAP3+: Clock: Adding ehci clock enabling

Adding ehci clock enabling mechanism part of clock framework.
When essential clocks are enabled during init phase usb host
clocks can also be enabled from clock framework.

Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Govindraj.R <govindraj.raja@ti.com>
Tested-by: Stefano Babic <sbabic@denx.de>