spi: ppc4xx: Fix fallout from include cleanup

The driver uses several symbols declared in <linux/platform_device.h>,
e.g module_platform_driver(). Include this header explicitly now that
<linux/of_platform.h> doesn't include <linux/platform_device.h> any
more.

Fixes: ef175b29

 ("of: Stop circularly including of_device.h and of_platform.h")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20240210164006.208149-6-u.kleine-koenig@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>