atmel-mci: Add support for inverted detect pin

Same patch as before, modified to use bool. Also adds description of
the new field in struct atmel_mci that I missed in the first patch.

This patch adds Atmel MCI support for inverted detect pins.

Signed-off-by: Jonas Larsson <jonas.larsson@martinsson.se>
Acked-by: Pierre Ossman <pierre@ossman.eu>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>