pwm: dwc: split pci out of core driver

Moving towards adding non-pci support for the driver, move the pci
parts out of the core into their own module. This is partly due to
the module_driver() code only being allowed once in a module and also
to avoid a number of #ifdef if we build a single file in a system
without pci support.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20230907161242.67190-2-ben.dooks@codethink.co.uk
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>