arm: Set the number of FloatRegs to zero.

arm: Set the number of FloatRegs to zero.

ARM no longer uses the floating point register file and uses the
vector registers instead. This avoids checkpointing a bunch of unused
registers, making it hard to tell where floating point instructions
are keeping their values, etc.

Change-Id: I23145ba750f1dd9ff5b815395e073c410120840d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22524
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>