atmel-boards: add missing atmel_mci.h

atmel-boards: add missing atmel_mci.h

commit 72fa467988e7944407a634ddc4bc6a2df685c04c moved atmel_mci_init() into
include/atmel_mci.h. Some AT91 boards are also using this interface and need
to include atmel_mci.h now.

This patch fixes MAKEALL complaints like this:

---8<---
Configuring for ethernut5 - Board: ethernut5, Options: AT91SAM9XE
ethernut5.c: In function 'board_mmc_init':
ethernut5.c:235:2: warning: implicit declaration of function 'atmel_mci_init' [-Wimplicit-function-declaration]
--->8---

Signed-off-by: Andreas Bießmann <biessmann@corscience.de>
CC: Albert Aribaud <albert.u.boot@aribaud.net>
CC: Reinhard Meyer <u-boot@emk-elektronik.de>
CC: egnite GmbH <info@egnite.de>