ARM: dts: DRA7: Add dt nodes for PWMSS

Add PWMSS device tree nodes for DRA7 SoC family and add documentation
for dt bindings.

Signed-off-by: Vignesh R <vigneshr@ti.com>
[fcooper@ti.com: Add eCAP and use updated bindings for PWMSS and ePWM]
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>