KVM: riscv: selftests: Add sstc timer test

Add a KVM selftests to validate the Sstc timer functionality.
The test was ported from arm64 arch timer test.

Signed-off-by: Haibo Xu <haibo1.xu@intel.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>