hwrng: ingenic - Replace of_device.h with explicit of.h include

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.

of_device.h isn't needed, but of.h is and was implicitly included by it.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>