libutee: support RISC-V syscall

Add 64-bit RISC-V TA system call function.
Modify the utee_syscalls_asm.S file to be generic.

Signed-off-by: liushiwei <liushiwei@eswincomputing.com>
Reviewed-by: ch

libutee: support RISC-V syscall

Add 64-bit RISC-V TA system call function.
Modify the utee_syscalls_asm.S file to be generic.

Signed-off-by: liushiwei <liushiwei@eswincomputing.com>
Reviewed-by: chenchaokai <chenchaokai@eswincomputing.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

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