mmc: atmel-mci: remove useless include

Definitions from linux/platform_data/atmel.h are not used, remove the
include.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>