ARM: at91/dt: remove useless usb clock

The ohci driver now calls clk_set_rate on the uhpck clock (which forwards
set_rate requests to its parent: the usb clock).
Remove useless references to usb clocks from ohci definitions.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>