pwm: jz4740: Document known limitations

The JZ4740 PWM implementation doesn't fulfill the (up to now
insufficiently documented) requirements of the PWM API. At least
document them in the driver.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-jz4740.c

@@ -2,6 +2,11 @@
 /*
  *  Copyright (C) 2010, Lars-Peter Clausen <lars@metafoo.de>
  *  JZ4740 platform PWM support
+ *
+ * Limitations:
+ * - The .apply callback doesn't complete the currently running period before
+ *   reconfiguring the hardware.
+ * - Each period starts with the inactive part.
  */
 
 #include <linux/clk.h>