hexagon: remove unnecessary prom.h includes

Hexagon does not have a prom.h header, so it is probably broken with
CONFIG_OF enabled. In any case, remove unnecessary prom.h include in
preparation to make prom.h optional.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Richard Kuo <rkuo@codeaurora.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Cc: linux-hexagon@vger.kernel.org