OvmfPkg/BaseMemEncryptSevLib: introduce MemEncryptSevClearMmioPageEncMask()
    
    BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275
    
    The MemEncryptSevClearMmioPageEncMask() helper can be used for clearing
    the memory encryption mask for the Mmio region.
    
    The MemEncryptSevClearMmioPageEncMask() is a simplified version of
    MemEncryptSevClearPageEncMask() -- it does not flush the caches after
    clearing the page encryption mask.
    
    Cc: James Bottomley <jejb@linux.ibm.com>
    Cc: Min Xu <min.m.xu@intel.com>
    Cc: Jiewen Yao <jiewen.yao@intel.com>
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
    Cc: Laszlo Ersek <lersek@redhat.com>
    Cc: Erdem Aktas <erdemaktas@google.com>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>
    Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
    Message-Id: <20210519181949.6574-10-brijesh.singh@amd.com>