KVM: PPC: Book3S: Add ONE_REG register names that were missed

Commit 3b783474 ("KVM: PPC: Book3S HV: Reserve POWER8 space in get/set_one_reg") added definitions for several KVM_REG_PPC_* symbols
but missed adding some to api.txt.  This adds them.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>