pwm: sun4i: Support direct clock output on Allwinner A64

Allwinner A64 is capable of a direct clock output on PWM (see A64 User
Manual chapter 3.10). Add support for this in the sun4i PWM driver.

Signed-off-by: Peter Vasil <peter.vasil@gmail.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>