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/board/altera/nios2-generic/custom_fpga.h b/board/altera/nios2-generic/custom_fpga.h
index f7f3853..fd3ec9a 100644
--- a/board/altera/nios2-generic/custom_fpga.h
+++ b/board/altera/nios2-generic/custom_fpga.h
@@ -51,6 +51,7 @@
 /* led_pio.s1 is a altera_avalon_pio */
 #define LED_PIO_BASE 0x82120870
 #define LED_PIO_WIDTH 8
+#define LED_PIO_RSTVAL 0x0
 
 /* high_res_timer.s1 is a altera_avalon_timer */
 #define CONFIG_SYS_TIMER_BASE 0x82120820