USB: ehci-atmel: add needed of.h header file

Compilation error in case of non-DT configuration without this
of.h header file.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>