pwm: Add a stub for devm_pwmchip_add()

The devm_pwmchip_add() can be called by a module that optionally
instantiates PWM chip. In the case of CONFIG_PWM=n, the compilation
can't be performed. Hence, add a necessary stub.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>