mmc: core: Update check for enhanced rpmb support

Enhnaced RPMB support is added in eMMC v5.1 and later.
This change uses correct ext_csd rev (0x8 for v5.1) to
set enhanced rpmb support.

Change-Id: I5f99223ff55b1831793066c432d04ed60823881b
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>