RISC-V: Provide the frequency of time CSR via hwprobe

The RISC-V architecture makes a real time counter CSR (via RDTIME
instruction) available for applications in U-mode but there is no
architected mechanism for an application to discover the frequency
the counter is running at. Some applications (e.g., DPDK) use the
time counter for basic performance analysis as well as fine grained
time-keeping.

Add support to the hwprobe system call to export the time CSR
frequency to code running in U-mode.

Signed-off-by: Yunhui Cui <[email protected]>
Reviewed-by: Evan Green <[email protected]>
Reviewed-by: Anup Patel <[email protected]>
Acked-by: Punit Agrawal <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

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