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

In the Linux kernel, the following vulnerability has been resolved:

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.