mx5: add iomux-mx51.h include

Allow usage of the imx-common/iomux-v3.h framework by including pad settings
for the i.MX51. The content of the file is taken from Linux kernel at
commit 5d23b39 plus the required changes to make it work in U-Boot.

The contained pad settings are the minimum required to make an Efika MX boot
and get all the currently-implemented peripherals working in U-Boot.

It is recommended that this file not be just a dumping ground for pins but
only contain the settings required for all the boards using it.

Changes for v2:
 * reference commit id from Linux kernel
 * additionally roll in the USB pads
 * removed GPIO_NUMBER define

Signed-off-by: Matt Sealey <matt@genesi-usa.com>
Acked-by: Stefano Babic <sbabic@denx.de>