avoid dereferencing NULL pointer in lib_arm/armlinux.c

* Patch by Stephen Williams, 15 Mar 2004:
  Fix CONFIG_SERIAL_SOFTWARE_FIFO documentation

* Patch by Tolunay Orkun, 15 Mar 2004:
  Initialize bi_opbfreq to real OPB frequency via get_OPB_freq()

* Patch by Travis Sawyer, 15 Mar 2004:
  Update CREDITS & MAINTAINERS files for PPC440GX & Ocotea port