pwm: Add support for Azoteq IQS620A PWM generator

This patch adds support for the Azoteq IQS620A, capable of generating
a 1-kHz PWM output with duty cycle between ~0.4% and 100% (inclusive).

Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>