pnv/xive2: Support for "OS LGS Push" TIMA operation

pnv/xive2: Support for "OS LGS Push" TIMA operation

Adds support for single byte writes to offset 0x15 of the TIMA address
space.  This offset holds the Logical Server Group Size (LGS) field.
The field is used to evenly distribute the interrupt load among the
members of a group, but is unused in the current implementation so we
just support the writing of the value for now.

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>