leds: leds-pwm: provide a common function to setup a single led-pwm device

Provide a common function to setup a single led-pwm device, replacing
the platform data initialisation path with this function.  This allows
us to have a common method of creating these devices in a consistent
manner, which then allows us to place the probe failure cleanup in one
place.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Bryan Wu <cooloney@gmail.com>