ARM: dts: am437x: Add missing compatibles to PWM binding documents

There are several SOC specific compatibles for ECAP, EHRPWM and PWMMS
that are in use but aren't properly documented. Therefore, fix this
by adding the compatibles to the appropriate binding documents.

While at it make minor corrections to the binding document.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>