i386/cpu: Update how the EBX register of CPUID 0x8000001F is set

i386/cpu: Update how the EBX register of CPUID 0x8000001F is set

Update the setting of CPUID 0x8000001F EBX to clearly document the ranges
associated with fields being set.

Fixes: 6cb8f2a663 ("cpu/i386: populate CPUID 0x8000_001F when SEV is active")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <5822fd7d02b575121380e1f493a8f6d9eba2b11a.1664550870.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>