riscv: Add Sstc extension support

Sstc extension allows the guest OS to program the timer directly without
relying on the SBI call. The kernel detects the presence of Sstc extnesion
from the riscv,isa DT property. Add the Sstc extension to the device tree
if it is supported by the host.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20220815101325.477694-5-apatel@ventanamicro.com


Signed-off-by: Will Deacon <will@kernel.org>