mmc: core: Add in support to expose PRV for v4 MMCs

The JEDEC MMC v4 spec defines a new PRV value in place of the original
fwrev and hwrev specified in v1. We can expose this in the kernel to enable
user space to more easily determine the product revision of a given MMC.

Signed-off-by: Bernie Thompson <bhthompson@chromium.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>