clocksource: ftm: Add FlexTimer Module (FTM) Timer devicetree Documentation

The FTM binding could be used on Vybrid and LS1+, add a binding
document for it.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Jingchang Lu <b35083@freescale.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>