arch-arm: CPTR.FPEN controlling SVE enablement

arch-arm: CPTR.FPEN controlling SVE enablement

CheckSveEnabled shouldn't check for .ZEN only.
SVE instructions require Advanced SIMD to be supported as
well (CPTR.FPEN) with the caveat of ZEN check having priority
over the FPEN.

Change-Id: Ia1b5f7df3e25e7ffcad472542cb973635f62637b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28768
Tested-by: kokoro <noreply+kokoro@google.com>