dt/powerpc: move of_bus_type infrastructure to ibmebus

arch/powerpc/kernel/ibmebus.c is the only remaining user of the
of_bus_type support code for initializing the bus and registering
drivers.  All others have either been switched to the vanilla platform
bus or already have their own infrastructure.

This patch moves the functionality that ibmebus is using out of
drivers/of/{platform,device}.c and into ibmebus.c where it is actually
used.  Also renames the moved symbols from of_platform_* to
ibmebus_bus_* to reflect the actual usage.

This patch is part of moving all of the of_platform_bus_type users
over to the platform_bus_type.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>