ppc4xx: Consolidate get_OPB_freq()

ppc4xx: Consolidate get_OPB_freq()

All 4xx variants had their own, mostly identical get_OPB_freq()
function. Some variants even only had the OPB frequency calculated
in this routine and not supplied the sys_info.freqOPB variable
correctly (e.g. 405EZ). This resulted in incorrect OPB values passed
via the FDT to Linux.

This patch now removes all those copies and only uses one function
for all 4xx variants (except for IOP480 which doesn't have an OPB).

Signed-off-by: Stefan Roese <sr@denx.de>