imx: imx6: perform gpr_init only on suitable cpu types

If the function gpr_init is used in a common MX6 spl
implementation we have to ensure that it is only called for
suitable cpu types, otherwise it breaks hardware parts like
enet1, can1, can2, etc.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.de>