lib: timer: Provide a hart based timer feature

As per the RISC-V specification, time value can be obtained from a time
CSR implemented in hardware or a MMIO based IP block. Qemu virt machine
already supports timer csr while CLINT provides the timer for other
platforms.

Implement a hart specific timer feature that can be detected at runtime.
As CSR based timer implementation are faster than MMIO address based, it
is always preferred over MMIO based one.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Tested-by: Jonathan Balkind <jbalkind@cs.princeton.edu>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

lib/sbi/sbi_hart.c

@@ -271,6 +271,12 @@ static void sbi_hart_detect_features(u32 hartid)
 			feature |= SBI_HART_HAS_MCOUNTEREN;
 	}
 
+	/* Detect if hart supports time CSR */
+	trap.cause = 0;
+	csr_read_allowed(CSR_TIME, (unsigned long)&trap);
+	if (!trap.cause)
+		feature |= SBI_HART_HAS_TIME;
+
 	sbi_hart_set_feature(hartid, feature);
 }