misc: Add ab8500 pwm driver

This patch adds a Pulse Width Modulation driver for Analog Baseband
Chip AB8500.

Signed-off-by: Arun Murthy <arun.murthy@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Acked-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>