ARM: at91: sama5 always uses DT

It makes no sense for sama5 support to be enabled if we don't
also enable USE_OF. Making this automatic in Kconfig avoids
a possible randconfig conflict between the old and new clock
support code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>