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/misc/efi.pandoc

@@ -104,6 +104,8 @@ Specifies an XSM module to load.
 
 Specifies a CPU microcode blob to load. (x86 only)
 
+Only available when Xen is compiled with CONFIG_MICROCODE_LOADING enabled.
+
 ###`dtb=<filename>`
 
 Specifies a device tree file to load.  The platform firmware may provide a

docs/misc/xen-command-line.pandoc

@@ -2751,9 +2751,10 @@ performance.
     Applicability: x86
     Default: `scan` is selectable via Kconfig, `nmi,digest-check`
 
-Controls for CPU microcode loading. For early loading, this parameter can
-specify how and where to find the microcode update blob. For late loading,
-this parameter specifies if the update happens within a NMI handler.
+Controls for CPU microcode loading, available when `CONFIG_MICROCODE_LOADING`
+is enabled. For early loading, this parameter can specify how and where to
+find the microcode update blob. For late loading, this parameter specifies if
+the update happens within a NMI handler.
 
 'integer' specifies the CPU microcode update blob module index. When positive,
 this specifies the n-th module (in the GrUB entry, zero based) to be used