PowerPC update comments for the MMA instruction name changes.

PowerPC update comments for the MMA instruction name changes.

The mnemonics for the pmxvf16ger*, pmxvf32ger*,pmxvf64ger*, pmxvi4ger8*,
pmxvi8ger4*, and pmxvi16ger2* instructions were officially changed to
pmdmxbf16ger*, pmdmxvf32ger*, pmdmxvf64ger*, pmdmxvi4ger8*, pmdmxvi8ger4*,
pmdmxvi16ger* respectively.  The old mnemonics are still supported by the
assembler as extended mnemonics.  The disassembler generates the new
mnemonics.  The name changes occurred in commit:

  commit bb98553cad4e017f1851153fa5de91f2cee98fb2
  Author: Peter Bergner <bergner@linux.ibm.com>
  Date:   Sat Oct 8 16:19:51 2022 -0500

    PowerPC: Add support for RFC02658 - MMA+ Outer-Product Instructions

    gas/
            * config/tc-ppc.c (md_assemble): Only check for prefix opcodes.
            * testsuite/gas/ppc/rfc02658.s: New test.
            * testsuite/gas/ppc/rfc02658.d: Likewise.
            * testsuite/gas/ppc/ppc.exp: Run it.

    opcodes/
            * ppc-opc.c (XMSK8, P_GERX4_MASK, P_GERX2_MASK, XX3GERX_MASK): New.
            (powerpc_opcodes): Add dmxvi8gerx4pp, dmxvi8gerx4, dmxvf16gerx2pp,
            dmxvf16gerx2, dmxvbf16gerx2pp, dmxvf16gerx2np, dmxvbf16gerx2,
            dmxvi8gerx4spp, dmxvbf16gerx2np, dmxvf16gerx2pn, dmxvbf16gerx2pn,
            dmxvf16gerx2nn, dmxvbf16gerx2nn, pmdmxvi8gerx4pp, pmdmxvi8gerx4,
            pmdmxvf16gerx2pp, pmdmxvf16gerx2, pmdmxvbf16gerx2pp, pmdmxvf16gerx2np,
            pmdmxvbf16gerx2, pmdmxvi8gerx4spp, pmdmxvbf16gerx2np, pmdmxvf16gerx2pn,
            pmdmxvbf16gerx2pn, pmdmxvf16gerx2nn, pmdmxvbf16gerx2nn.

This patch updates the comments in the various gdb files to reflect the
name changes.  There are no functional changes made by this patch.

The older instruction names are still used in the test
gdb.reverse/ppc_record_test_isa_3_1.exp for backwards compatibility.

Patch has been tested on Power 10 with no regressions.