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.deSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-ppc4xx.c

@@ -31,6 +31,7 @@
 #include <linux/of_platform.h>
 #include <linux/interrupt.h>
 #include <linux/delay.h>
+#include <linux/platform_device.h>
 
 #include <linux/spi/spi.h>
 #include <linux/spi/spi_bitbang.h>