ARM: i.MX: mx21ads: Replace direct handling of peripheral bits with GPIOs

This patch replaces direct handling of peripheral bits with GPIOs by
using basic-mmio-gpio driver. As a result this change eliminate some
low-level code and removes custom io_map() from the board.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>