dt-bindings: clock: Move at91.h to dt-bindigs/clock

Most of the clock related dt-binding header files are located in
dt-bindings/clock folder. It would be good to keep all the similar
header files at a single location.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
CC: Rob Landley <rob@landley.net>
CC: Andrew Victor <linux@maxim.org.za>
CC: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Acked-by: Boris BREZILLON <b.brezillon.dev@gmail.com>
[nicolas.ferre@atmel.com: add new at91sam9261 & at91sam9rl]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>