pwm: Enable Kona PWM to be built for the Cygnus architecture

The Cygnus architecture uses a Kona PWM. This is already present in the
device tree but can't be built actually. Hence, allow the Kona PWM to
be built for the Cygnus architecture.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>