mips: bmips: BCM6358: make sure CBR is correctly set

mips: bmips: BCM6358: make sure CBR is correctly set

It was discovered that some device have CBR address set to 0 causing
kernel panic when arch_sync_dma_for_cpu_all is called.

This was notice in situation where the system is booted from TP1 and
BMIPS_GET_CBR() returns 0 instead of a valid address and
!!(read_c0_brcm_cmt_local() & (1 << 31)); not failing.

The current check whether RAC flush should be disabled or not are not
enough hence lets check if CBR is a valid address or not.

Fixes: ab327f8acdf8 ("mips: bmips: BCM6358: disable RAC flush for TP1")
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>