After moving SOC_IMX51 support over to device tree, all i.MX5 support
becomes device tree only now.  So options SOC_IMX5 and SOC_IMX51 can
just be under 'Device tree only'.

While at it, 'select ARCH_MXC_IOMUX_V3' is dropped, since it's only
needed by non-DT build before.
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>