atmel_sdhci: Force card-detect if MMC_CAP_NONREMOVABLE.

If the device attached to the MMC bus is not removable, set force card-detect
bit to bypass card detection procedure, so card detection pin can be used for
other purposes.

It's also a workaround for SAMA5D2 who doesn't drive CMD if using GPIO for card
detection.

Signed-off-by: Zixun LI <zli@ogga.fr>
Reviewed-by: Eugen Hristev <eugen.hristev@collabora.com>