pwm: dt: Fix description of second PWM cell

The second cell in the PWM specifier denotes the period in nanoseconds,
not the duty cycle. The latter can be freely configured at runtime and
a PWM with a fixed duty cycle would be rather pointless.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: "Benoît Thébaudeau" <benoit.thebaudeau@advansee.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>