KVM: riscv: selftests: Add SBI DBCN extension to get-reg-list test

We have a new SBI debug console (DBCN) extension supported by in-kernel
KVM so let us add this extension to get-reg-list test.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>