pwm: dwc: Move memory allocation to own function

In preparation for adding other bus support, move the allocation
of the PWM structure out of the main driver code.

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>