ppc/xive2: Allow 1-byte write of Target field in TIMA

ppc/xive2: Allow 1-byte write of Target field in TIMA

When running PowerVM, the console is littered with XIVE traces regarding
invalid writes to TIMA address 0x100b6 due to a lack of support for writes
to the "TARGET" field which was added for XIVE GEN2.  To fix this, we add
special op support for 1-byte writes to this field.

Signed-off-by: Glenn Miles <milesg@linux.vnet.ibm.com>
Signed-off-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>