nios2: add gpio support

This patch adds driver for a trivial gpio core, which is described
in http://nioswiki.com/GPIO

. It is used for gpio led and nand flash
interface in u-boot.

When CONFIG_SYS_GPIO_BASE is not defined, board may provide
its own driver.

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>