staging: greybus: Add TODO item about modernizing the pwm code

drivers/staging/greybus/pwm.c uses the old style PWM callbacks, new drivers
should stick to the atomic API instead.

Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Link: https://lore.kernel.org/r/20201208101607.42785-1-uwe@kleine-koenig.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>