pwm: add period limits, warn over syslog

Added minimum, maximum and default period settings to board definitions
PWM will now have a default period as defined in the board defintion.
When using pwm_write() writing 1.0f or above will default to 100%.

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

src/intel_de3815.c

@@ -43,6 +43,9 @@ mraa_intel_de3815()
     b->aio_count = 0;
     b->adc_raw = 0;
     b->adc_supported = 0;
+    b->pwm_default_period = 500;
+    b->pwm_max_period = 2147483;
+    b->pwm_min_period = 1;
 
     b->pins = (mraa_pininfo_t*) malloc(sizeof(mraa_pininfo_t)*MRAA_INTEL_DE3815_PINCOUNT);