pwm: atmel: Change prototype of a helper to prepare further changes

This prepares the driver for further changes that will make it harder to
determine the pwm_chip from a given atmel_pwm_chip. To just not have to
do that, rework atmel_pwm_enable_clk_if_on() to take a pwm_chip.

Link: https://lore.kernel.org/r/c9a92f77760e401debfe0c9bfc086222f31fb3c4.1707900770.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>