[V3 0/7]Add i.MX51 3-stack board support

[Jason Wang]

I don't know why 2 patches([patch 2/3] and [patch 3/3]) are lost in
the V2 review.
In V3, i fix all comments both in V1 and V2.

Big diffs:
1) the number of patches increases from 3 to 7, because i add 1 3ds
common debug board patch, 2 iomux pad rename and update patches and
1 revert mx31_3ds debug board relating stuffs patch.

2) Though the smsc911x_platform_config is a little bit different
between mx31_3ds and mx51_3ds originally,  when using the common
configuration in the mxc debug board, they all works fine. I have
validated these two BSPs through mounting NFS rootfs.

3) the patch [3/7] is a rename and cleanup of iomux pad definition,
i don't know if this update has been merged, if so, please drop
it.