pwm: fixed wrong line ending being checked by strtol

Move from fopen to POSIX open introduced this bug in strtol

Signed-off-by: Maximilian von Tschirschnitz <maximilian.tschirschnitz@intel.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
contributing.md: add a few contribution rules matching UPM

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
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>