target/riscv: Reuse the conversion function of priv_spec

target/riscv: Reuse the conversion function of priv_spec

Public the conversion function of priv_spec in cpu.h, so that tcg-cpu.c
could also use it.

Signed-off-by: Jim Shu <jim.shu@sifive.com>
Signed-off-by: Fea.Wang <fea.wang@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240606135454.119186-2-fea.wang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>