firmware: microchip: Replace of_device.h with explicit include

The DT of_device.h and of_platform.h date back to the separate
of_platform_bus_type before it was merged into the regular platform bus.
As part of that merge prepping Arm DT support 13 years ago, they
"temporarily" include each other. They also include platform_device.h
and of.h. Soon the implicit includes are going to be removed.

of_device.h isn't needed, but platform_device.h is.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>