openrisc: clean-up prom.h

Clean-up some copy/paste declarations that are not necessary. All the
functions either don't exist or are already declared in other headers.
This is needed in preparation of of_irq.h clean-up.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: linux@lists.openrisc.net