imx: mx27 move GPIO_PORTx to gpio.h

These GPIO_PORTx macros should be in gpio.h, but not in imx-regs.h.

Also, imx-regs.h and iomux-v3.h has same macro defintion for
GPIO_PORTx, and both of them are included in mxc_i2c.c(include
mxc_i2c.h). This will incur build warnings with macro redefinition.

Since iomux-v3.h is not compatible with mx27, we can not simply
include iomux-v3.h for mx27, so move the GPIO_PORTx to gpio.h to
fix the build warning.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>