mmc: host: atmel-mci: Add support for non-removable slots

Add support for non-removable slots which have no card detection GPIO
and which should not be polled for a card change.

Signed-off-by: Timo Kokkonen <timo.kokkonen@offcode.fi>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>