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>

Documentation/mmc/mmc-dev-attrs.txt

@@ -22,6 +22,7 @@ All attributes are read-only.
 	manfid			Manufacturer ID (from CID Register)
 	name			Product Name (from CID Register)
 	oemid			OEM/Application ID (from CID Register)
+	prv			Product Revision (from CID Register) (SD and MMCv4 only)
 	serial			Product Serial Number (from CID Register)
 	erase_size		Erase group size
 	preferred_erase_size	Preferred erase size