Document brcm,bmips-cbr-reg property.

Some SoC suffer from a BUG where CBR(Core Base Register)
address might be badly or never initialized by the Bootloader
or reading it from co-processor registers, if the system boots
from secondary CPU, results in invalid address.

The CBR address is always the same on the SoC.

Usage of this property is to give an address also in these broken
configuration/bootloader.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>