mmc: block: support no access to boot partitions

Intel Medfield platform blocks access to eMMC boot partitions which
results in switch errors.  Since there is no access, mmcboot0/1
devices should not be created.  Add a host capability to reflect that.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>