bus: brcmstb_gisb: Look up register offsets in a table

There are at least 4 incompatible variations of this hardware block,
so let's use the ARB_* constants as a table index instead of hardcoding
specific register offsets.  Also, allow for the possibility of adding
old devices that are missing some of the registers.

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