gpio: Add driver for Altera's PIO core

This driver may handle multiple PIO cores and thus needs to be
setup by calling the altera_pio_init() function within the early
board setup routine.

The driver comes with some extras, see below the copyleft header.

Signed-off-by: Joachim Foerster <joachim.foerster@missinglinkelectronics.com>
Tested-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
diff --git a/drivers/gpio/Makefile b/drivers/gpio/Makefile
index f505813..b5264d1 100644
--- a/drivers/gpio/Makefile
+++ b/drivers/gpio/Makefile
@@ -35,6 +35,7 @@
 COBJS-$(CONFIG_S5P)		+= s5p_gpio.o
 COBJS-$(CONFIG_TEGRA2_GPIO)	+= tegra2_gpio.o
 COBJS-$(CONFIG_DA8XX_GPIO)	+= da8xx_gpio.o
+COBJS-$(CONFIG_ALTERA_PIO)	+= altera_pio.o
 
 COBJS	:= $(COBJS-y)
 SRCS 	:= $(COBJS:.o=.c)