cpu: 83xx: Add missing dependency on CPU_MPC83XX

It looks quite weird that for non PPC platforms cpu driver for MPC83xx can
be selected. That's why define proper dependency.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>