USB: at91: Add USB EHCI driver for at91sam9g45 series

Add host USB High speed driver for at91sam9g45 series.
The host driver is an EHCI with its companion OHCI. EHCI is
handled by the new ehci-atmel.c whereas the OHCI is always
handled by ohci-at91.c.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>