No pwmout desctructor

[LMESTM]

In hal/hal/pwmout_api.h there is the definition pwmout_free for targets to implement.
void pwmout_free(pwmout_t *obj);
Many targets actually do have an implementation of this function, but this is never call from PwmOut class as there is no ~PwmOut.
Note: This is also valid for other objects, but I trying to test this one.

[LMESTM]

I have a proposal in a branch that I tested ok on STM32
LMESTM@7a64b23
but I amnot sure it's the proper implementation - so feedback would be welcome before a possible PR is sent. If not the proper way, I'd prefer mbed staff to address the issue.

[ciarmcom]

ARM Internal Ref: IOTMORF-150