pwm: sifive: Add DT documentation for SiFive PWM Controller

DT documentation for PWM controller added.

Signed-off-by: Wesley W. Terpstra <wesley@sifive.com>
[Atish: Compatible string update]
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>