x86/ucode: Add Kconfig option to remove microcode loading

Amend docs to reflect ucode= command/stanza depend on MICROCODE_LOADING
being set.

The new MICROCODE_OP() is a conditional setter for the microcode_ops
struct. By using IS_ENABLED() there rather than ifdef we allow DCE to
remove all statics no longer used when microcode loading is disabled
without tagging them with __maybe_unused.

Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

docs/admin-guide/microcode-loading.rst

@@ -23,6 +23,8 @@ TSX errata which necessitated disabling the feature entirely), or the addition
 of brand new features (e.g. the Spectre v2 controls to work around speculative
 execution vulnerabilities).
 
+Microcode loading support in Xen is controlled by the
+``CONFIG_MICROCODE_LOADING`` Kconfig option.
 
 Boot time microcode loading
 ---------------------------