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 <breno.lima@nxp.com>
Acked-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>