of: only include prom.h on sparc

The dependency on prom.h by the core DT code is now removed and only
sparc needs to include prom.h for the core code.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Git-commit: b5b4bb3f6a11f9c37b6d53138244f2ffe5bacd12
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Ian Maund <imaund@codeaurora.org>