mmc: card: Do not scan RPMB partitions

Do not scan rpmb partitions for "soft" partitions, since the rpmb
partition contains protected data. Silences the following message
during boot:

 mmcblkXRPMB: unknown partition table

Signed-off-by: Johan Rudholm <johan.rudholm@stericsson.com>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Krishna Konda <kkonda@codeaurora.org>
Signed-off-by: Chris Ball <cjb@laptop.org>