microblaze: clean-up prom.h implicit includes

While powerpc is a mess of implicit includes by prom.h, microblaze just
copied this and is easily fixed. Add the necessary explicit includes and
remove unnecessary includes and other parts from prom.h

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: microblaze-uclinux@itee.uq.edu.au
Cc: netdev@vger.kernel.org