mmc: core: expose RPMB partition only for CMD23 capable hosts

SET_BLOCK_COUNT CMD23 is needed for all access to RPMB partition.  If
block count is not set by CMD23, all subsequent read/write commands fail
as per eMMC specification. So, If the host does not support CMD23, do not
expose RPMB partition.

Accessing RPMB partition can cause hang / huge delay for hosts which do
not support CMD23.

Signed-off-by: Balaji T K <balajitk@ti.com>
Reported-and-Tested-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>