boards: mec15xxevb_assy6853: Enable pwm sanitycheck

Enable following sanitychecks

mec15xxevb_assy6853/samples/basic/servo_motor/sample.servo_motor
mec15xxevb_assy6853/tests/drivers/pwm/pwm_api/peripheral.pwm

Run with: sanitycheck -t pwm -p mec15xxevb_assy6853

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>