KVM: riscv: selftests: Split SBI FWFT into separate feature-specific sublists

KVM: riscv: selftests: Split SBI FWFT into separate feature-specific sublists

Divide the monolithic SBI FWFT (Firmware Features) register list into
separate sublists, each testing a specific FWFT feature independently
with proper dependency checking.

Previously, all FWFT features were tested together in a single sublist.
This caused issues because:
1. Not all FWFT features are available on all platforms
2. Some features depend on specific ISA extensions (e.g., pointer_masking
requires Smnpm)
3. Tests would fail if any single feature was unavailable

Add the feature-specific SBI FWFT sublists with the following
improvements:
- Add check_fwft_feature() helper to verify FWFT feature availability
  at runtime
- Update filter_reg() to handle per-feature FWFT register filtering

Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260601-kvm-get_reg_list-v2-v5-5-415d08a2813b@sifive.com
Signed-off-by: Anup Patel <anup@brainfault.org>