DesignWare PWM support for device-tree probing

[Ben Dooks]

This series is tidying up and adding device-tree support for the
DesignWare DW-APB-timers block.

Changes:

v3:
- change the compatible name
- squash down pwm count patch
- fixup patch naming

v2:
- fix #pwm-cells count to be 3
- fix indetation 
- merge the two clock patches
- add HAS_IOMEM as a config dependency