cmd/sbi: add implementation ID 6 - Coffer

The sbi command displays the ID of the implementation of the RISC-V
Supervisor Binary Interface Specification. A new ID for Coffer has recently
been added.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>