x86/cpu: Move MWAIT leaf definition to common header

Begin constructing a common place to keep all CPUID leaf definitions.
Move CPUID_MWAIT_LEAF to the CPUID header and include it where
needed.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/all/20241213205028.EE94D02A%40davehans-spike.ostc.intel.com