imx: imx6: Move gpr_init() function to soc.c

imx: imx6: Move gpr_init() function to soc.c

Since the gpr_init() function is common for boards using MX6S, MX6DL, MX6D,
MX6Q and MX6QP processors move it to the soc.c file.

Signed-off-by: Breno Lima <[email protected]>
Acked-by: Stefano Babic <[email protected]>
Reviewed-by: Lukasz Majewski <[email protected]>
Reviewed-by: Fabio Estevam <[email protected]>