mips: bmips: setup: make CBR address configurable

Add support to provide CBR address from DT to handle broken
SoC/Bootloader that doesn't correctly init it. This permits to use the
RAC flush even in these condition.

To provide a CBR address from DT, the property "brcm,bmips-cbr-reg"
needs to be set in the "cpus" node. On DT init, this property presence
will be checked and will set the bmips_cbr_addr value accordingly. Also
bmips_rac_flush_disable will be set to false as RAC flush can be
correctly supported.

The CBR address from DT will overwrite the cached one and the
one set in the CBR register will be ignored.

Also the DT CBR address is validated on being outside DRAM window.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>