[MIPS] Add CFE support to BCM47XX

Add CFE support to the BCM47XX code.  That includes querying CFE environment
variables as well as using CFE to print messages before the serial port is
initialized (early printk).
    
Signed-off-by: Aurelien Jarno <aurel32@farad.aurel32.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>