Clean up header files in RMI platform code (sys/mips/rmi), and remove

Clean up header files in RMI platform code (sys/mips/rmi), and remove
unused files.

- remove clock.c and clock.h, these are not used after the new timer
  code was added.
- remove duplicated include files, fix header file ordering, remove
  some unneeded includes.
- rename mips/rmi/shared_structs.h which contains the RMI boot loader
  interface to mips/rmi/rmi_boot_info.h. Remove unused files
  mips/rmi/shared_structs_func.h and sys/mips/rmi/shared_structs_offsets.h
- merge mips/rmi/xlrconfig.h and mips/rmi/rmi_mips_exts.h, and remove
  duplicated functions.
- nlge - minor change to remove unneeded argument.
- Add FreeBSD svn keyword for headers