pwm: stmpe: Allow to compile as a module

pwm-stmpe is the only driver that cannot be built as a module. Add the
necessary boilerplate to also make this driver modular.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20250215143723.636591-2-u.kleine-koenig@baylibre.com


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>