pwm.c: cache period

There is an issue that  mraa_pwm_write would fail, because mraa_pwm_read
would fail and return 0.  When the read fails, journalctl shows an
error:
Dec 28 18:01:38 Edison libmraa[365]: pwm: Error in reading period

So now trying version, where the pwm object caches the period, that is
updated whenever you do a read or write of the period.  Side benefit is
that the write should be sped up.

Signed-off-by: Kurt Eckhardt <kurte@rockisland.com>
Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

include/mraa_internal_types.h

@@ -71,6 +71,7 @@ struct _pwm {
     int pin; /**< the pin number, as known to the os. */
     int chipid; /**< the chip id, which the pwm resides */
     int duty_fp; /**< File pointer to duty file */
+    int period;  /**< Cache the period to speed up setting duty */
     mraa_boolean_t owner; /**< Owner of pwm context*/
     /*@}*/
 };