target/ppc: Correct RMLS table

The table of RMA limits based on the LPCR[RMLS] field is slightly wrong.
We're missing the RMLS == 0 => 256 GiB RMA option, which is available on
POWER8, so add that.

The comment that goes with the table is much more wrong.  We *don't* filter
invalid RMLS values when writing the LPCR, and there's not really a
sensible way to do so.  Furthermore, while in theory the set of RMLS values
is implementation dependent, it seems in practice the same set has been
available since around POWER4+ up until POWER8, the last model which
supports RMLS at all.  So, correct that as well.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>

target/ppc/mmu-hash64.c

@@ -762,15 +762,16 @@ static target_ulong rmls_limit(PowerPCCPU *cpu)
 {
     CPUPPCState *env = &cpu->env;
     /*
-     * This is the full 4 bits encoding of POWER8. Previous
-     * CPUs only support a subset of these but the filtering
-     * is done when writing LPCR.
+     * In theory the meanings of RMLS values are implementation
+     * dependent.  In practice, this seems to have been the set from
+     * POWER4+..POWER8, and RMLS is no longer supported in POWER9.
      *
      * Unsupported values mean the OS has shot itself in the
      * foot. Return a 0-sized RMA in this case, which we expect
      * to trigger an immediate DSI or ISI
      */
     static const target_ulong rma_sizes[16] = {
+        [0] = 256 * GiB,
         [1] = 16 * GiB,
         [2] = 1 * GiB,
         [3] = 64 * MiB,