mx31: Introduce mx31_set_gpr function

Introduce mx31_set_gpr function for setting the GPR (General Purpose Register) on MX31.

This function can be useful for setting a group of pins into tied to some specific peripherals.

Reuse this function from the linux kernel.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>