spi: mpc52xx: explicitly include linux/platform_device.h

Since linux/of_platform.h had included linux/platform_device.h
and since that inclusion was removed, this driver now needs to include
the latter header file explicitly to prevent build errors:

drivers/spi/spi-mpc52xx.c: In function 'mpc52xx_spi_probe':
drivers/spi/spi-mpc52xx.c:396:20: error: invalid use of undefined type 'struct platform_device'

and more like that.

Fixes: 0d18bcdebb2f ("of: Stop circularly including of_device.h and of_platform.h")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-spi@vger.kernel.org
Link: https://lore.kernel.org/r/20231208185927.14124-1-rdunlap@infradead.orgSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-mpc52xx.c

@@ -22,6 +22,7 @@
 #include <linux/slab.h>
 #include <linux/of_address.h>
 #include <linux/of_irq.h>
+#include <linux/platform_device.h>
 
 #include <asm/time.h>
 #include <asm/mpc52xx.h>