pwm: bcm-iproc: Update dependencies for compile-test

Add dependency on COMMON_CLK and allow COMPILE_TEST for broader compile
coverage. Default to Y for IPROC SoCs. This allows the driver to simply
be enabled by selecting PWM.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
[thierry.reding@gmail.com: reword commit message]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>