ARM: OMAP: AM33xx hwmod: Add parent-child relationship for PWM subsystem

As part of PWM subsystem integration, PWM subsystem are sharing
resources like clock across submodules (ECAP, EQEP & EHRPWM). To handle
resource sharing & IP integration  rework on parent child relation
between PWMSS and ECAP, EQEP & EHRPWM child devices to support runtime PM.

Signed-off-by: Philip Avinash <avinashphilip@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>