soc: apple: mailbox: Add explicit include of platform_device.h

The DT of_device.h and of_platform.h date back to the separate
of_platform_bus_type before it as merged into the regular platform bus.
As part of that merge prepping Arm DT support 13 years ago, they
"temporarily" include each other and pull in various other headers. In
preparation to fix this, adjust the includes for what is actually needed.

platform_device.h is implicitly included by of_platform.h, but that's going
to be removed.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Signed-off-by: Hector Martin <marcan@marcan.st>