spi: spi-ppc4xx: include missing platform_device.h

spi: spi-ppc4xx: include missing platform_device.h

the driver currently fails to compile on 6.8-rc3 due to:
| spi-ppc4xx.c: In function ‘spi_ppc4xx_of_probe’:
| @346:36: error: invalid use of undefined type ‘struct platform_device’
| 346 |         struct device_node *np = op->dev.of_node;
|     |                                    ^~
| ... (more similar errors)

it was working with 6.7. Looks like it only needed the include
and its compiling fine!

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Link: https://lore.kernel.org/r/3eb3f9c4407ba99d1cd275662081e46b9e839173.1707490664.git.chunkeey@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>