ARM: keystone2: Cleanup SoC detection

Add proper register definition for JTAG ID and
cleanup cpu_is_* functions.

Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>