pwm: Add a helper to prepare a new PWM state

The pwm_init_state() helper prepares a new state object containing the
current PWM state except for the polarity and period fields which are
set to the reference values (those in struct pwm_args).

This is particularly useful for PWM users who want to apply a new duty-
cycle expressed relatively to the reference period without changing the
enable state.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>