target/riscv: clarify usage of `coreid`

By definition in `target/target.h`, `coreid` is not a unique identifier
of a target -- it can be the same for targets on different TAPs.

Change-Id: Ifce78da55fffe28dd8b6b06ecae7d8c4e305c0a2
Signed-off-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>