mmc: Atmel host kconfig cleanup for everyone else

This prevents those without an Atmel chip having a line in kernel
configuration which says "Atmel SD/MMC Driver" without any option.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>