pwm: lpss: Add support for PCI devices

Not all systems enumerate the PWM devices via ACPI. They can also be
exposed via the PCI interface.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Chew, Chiau Ee <chiau.ee.chew@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>