x86/speculation: Add a common function for MD_CLEAR mitigation update

Processor MMIO Stale Data mitigation uses similar mitigation as MDS and
TAA. In preparation for adding its mitigation, add a common function to
update all mitigations that depend on MD_CLEAR.

  [ bp: Add a newline in md_clear_update_mitigation() to separate
    statements better. ]

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>