usb: ehci: mv_ehci: remove unused clock

The origianl understanding of clock is wrong. The EHCI controller
only have one clock input.
Passing clock name by pdata is wrong. The clock is defined by device
iteself.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>