MdeModulePkg/EmmcDxe: Don't expose BlockIo interface for RPMB partition

This change is to avoid UEFI SCT failure as UEFI SCT has no knowledge
about how to accessing a EMMC RPMB partition.

The user needs to access RPMB partition should get access through
EFI_SD_MMC_PASS_THRU protocol with authentication key & mac.

Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>