lib: sbi: Implement Sstc extension

Recently, Sstc extension was ratified. It defines stimecmp which allows
the supervisor mode to directly update the timecmp value without the
need of the SBI call. The hardware also can inject the S-mode timer
interrupt direclty to the supervisor without going through the M-mode.
To maintain backward compatibility with the older software, SBI call
now uses stimecmp directly if the hardware supports.

Implement the Sstc extension.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

include/sbi/riscv_encoding.h

@@ -324,6 +324,10 @@
 #define CSR_STVAL			0x143
 #define CSR_SIP				0x144
 
+/* Sstc extension */
+#define CSR_STIMECMP			0x14D
+#define CSR_STIMECMPH			0x15D
+
 /* Supervisor Protection and Translation */
 #define CSR_SATP			0x180