libutee: Implement RISC-V helper functions for TA libraries

To enable ftrace support in TA libraries, the timer related functions
should be implemented. Add riscv_user_sysreg.h which implements thes

libutee: Implement RISC-V helper functions for TA libraries

To enable ftrace support in TA libraries, the timer related functions
should be implemented. Add riscv_user_sysreg.h which implements these
functions for TA libraries. The code is referenced from core header and
the M-mode related code is removed.

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Marouene Boubakri <marouene.boubakri@nxp.com>

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