core: riscv: provide arch_scall.h

Specifies what registers from thread_scall_regs are used for system calls.
The syscall number is provided in t0 register. The syscall max args is
provided in t1 reg

core: riscv: provide arch_scall.h

Specifies what registers from thread_scall_regs are used for system calls.
The syscall number is provided in t0 register. The syscall max args is
provided in t1 register. The return value is provided in a0 register.
The panic and panic code are provided respectively in a1 and a2 registers.

Signed-off-by: Marouene Boubakri <marouene.boubakri@nxp.com>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>

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