USB: ehci-atmel: add usb_clk for transition to CCF

The AT91 PMC (Power Management Controller) provides a USB clock used by
the different USB controllers (ehci, ohci and udc).
The atmel-ehci driver must configure the usb clock rate to 48Mhz in order
to get a fully functionnal USB host controller.
This configuration was formely done in mach-at91/clock.c, but will be
bypassed when moving to common clk framework.

This patch adds support for usb clock retrieval and configuration only if
CCF is enabled (CONFIG_COMMON_CLK).

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>