[MIPS] Move CFE code into arch/mips/fw/cfe

Move the platform independent part of the CFE code to arch/mips/fw/cfe from
arch/mips/sibyte/cfe.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>