mmc: core: Detect eMMC v4.5 ext_csd entries

The eMMC v4.5 Spec is released now:

EXT_CSD_REV	Extended CSD Revision
255-7		Reserved
6		Revision 1.6 (for MMC v4.5)
5		Revision 1.5 (for MMV v4.41)
...

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>