ARM: imx5: add step clock, used when reprogramming PLL1

This is the bypass clock used to feed the ARM partition
while we reprogram PLL1 to another rate.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>