leds: pwm: Support ACPI via firmware-node framework

leds: pwm: Support ACPI via firmware-node framework

DT specific handling is replaced by firmware-node abstration to support
ACPI specification of PWM LEDS.

Example ASL:
Device (PWML)
{
    Name (_HID, "PRP0001")
    Name (_DSD, Package () {
          ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
          Package () { Package () {"compatible",
                                    Package () {"pwm-leds"}}}})

    Device (PWL0)
    {
        Name (_HID, "PRP0001")
        Name (_DSD, Package () {
              ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
              Package () {
                           Package () {"label", "alarm-led"},
                           Package () {"pwms", Package ()
                                       {\_SB_.PCI0.PWM, 0, 600000, 0}},
                           Package () {"linux,default-state", "off"}}})
    }
}

Signed-off-by: Nikolaus Voss <nikolaus.voss@loewensteinmedical.de>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>