x86/ucode: Move interface from processor.h to microcode.h

x86/ucode: Move interface from processor.h to microcode.h

This reduces the complexity of processor.h, particularly the need to include
public/xen.h.  Substitute processor.h includes for microcode.h in some
sources, and add microcode.h includes in others.

Only 4 of the function declarations are actually called externally.  Move the
vendor init declarations to private.h

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>