USB: phy: Set default USB system clock to 80MHz

Clock driver increase the USB system clock under
nominal range from 80MHz to 100Mhz. Phy drivers
use the max clock rate to be set during probe.

Set the default clock rate at 80MHz so that it
can be increased only when 100Mhz is needed.

Change-Id: Ib0fa07bb7af5909be0f6c2f4d68e66412fdfe130
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>