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>
diff --git a/arch/arm/include/asm/arch-mx31/clock.h b/arch/arm/include/asm/arch-mx31/clock.h
index 2e3bce2..3e22d9a 100644
--- a/arch/arm/include/asm/arch-mx31/clock.h
+++ b/arch/arm/include/asm/arch-mx31/clock.h
@@ -37,6 +37,7 @@
 extern u32 imx_get_uartclk(void);
 extern void mx31_gpio_mux(unsigned long mode);
 extern void mx31_set_pad(enum iomux_pins pin, u32 config);
+extern void mx31_set_gpr(enum iomux_gp_func gp, char en);
 
 void mx31_uart1_hw_init(void);
 void mx31_spi2_hw_init(void);