thermal: loongson2: Replace of_device.h with explicit includes

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. They also include platform_device.h
and of.h.

of_device.h isn't needed, but mod_devicetable.h and property.h were
implicitly included.

Signed-off-by: Rob Herring <robh@kernel.org>