ARM: imx: Remove mx508 support

Only mx508 based board is mach-mx50_rdp and it has been marked as BROKEN
for several releases.

mx508 currently lacks clock support.

In case someone needs to add mx508 support back, then the recommended approach
is to use device tree.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>