mmc: mxs-mmc: add broken-cd property

According to bindings documentation for mmc, the property 'broken-cd'
can be used to indicate card-detection is not available and polling
must be used instead. This patch retrieves this property
and sets a custom flag. On the get_cd() hook, it returns 1 if
the flag is set, to always assume the card is present.

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>