mmc: core: Extend sysfs with OCR register

Registers CID and CSD are already exported through sysfs so let's make
this interface complete by adding missing OCR register.

Signed-off-by: Bojan Prtvar <prtvar.b@gmail.com>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>