gpio: stmpe: fully use convert probe to device-managed

The IRQ is registered via devm_request_threaded_irq(), making the driver
only partially device-managed. This changeset converts the entire driver
to using only devres APIs.

This change also removes platform_set_drvdata() since the information is
never retrieved to be used in the driver.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
[Bart: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>