ARM: keystone: remove unnecessary prom.h include

Remove unnecessary prom.h include in preparation to make prom.h optional.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>