soc: bcm: brcmstb: biuctrl: Tune 7260 BIU interface

7260A0 and B0 are both supported, and 7260A0 has a small difference in
that it does not support the write-back control register, which is why
we have a different array of registers. Update the comment above
b53_cpubiuctrl_no_wb_regs to denote that difference.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>