usb: ehci-omap: update clock names to be more generic

Rename usbhost2_120m_fck to usbhost_hs_fck and usbhost1_48m_fck
to usbhost_fs_fck, to better reflect the clocks' functionalities.

In OMAP4, the frequencies for the corresponding clocks are not
necessarily the same as with OMAP3, however the functionalities
are.

Signed-off-by: Keshava Munegowda <keshava_mgowda@ti.com>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>