arch-arm: Add support for Arm SVE fmmla instruction.

Add support for the Arm SVE Floating Point Matrix Multiply-Accumulate
(FMMLA) instruction. Both 32-bit element (single precision) and 64-bit
element (double precision) encodings are implemented, but because the
associated required instructions (LD1RO*, etc) have not yet been
implemented, the SVE Feature ID register 0 (ID_AA64ZFR0_EL1) has only
been updated to indicate 32-bit element support at this time.

For more information please refer to the "ARM Architecture Reference
Manual Supplement - The Scalable Vector Extension (SVE), for ARMv8-A"
(https://developer.arm.com/architectures/cpu-architecture/a-profile/
docs/arm-architecture-reference-manual-supplement-armv8-a)

Additional Contributors: Giacomo Travaglini

Change-Id: If3547378ffa48527fe540767399bcc37a5dab524
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70726
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
12 files changed