nios2: add gpio support to nios2-generic board

This patch adds gpio support of Altera PIO component to the
nios2-generic board. Though it drives only gpio_led at the
moment, it supports bidirectional port to control bit-banging
I2C, NAND flash busy status or button switches, etc.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Tested-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>