KVM: riscv: selftests: Refactor ISA and SBI extension sublist macros

KVM: riscv: selftests: Refactor ISA and SBI extension sublist macros

Refactor the get-reg-list test to use unified sublist macros for ISA
and SBI extensions, eliminating code duplication and improving
maintainability.

Previously, each extension had its own hand-coded sublist definition
(e.g., SUBLIST_ZICBOM, SUBLIST_AIA, etc.) and the config structures
repeated the same pattern. This made the code verbose and error-prone.

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-v5-4-415d08a2813b@sifive.com
Signed-off-by: Anup Patel <anup@brainfault.org>