bus: brcmstb_gisb: Honor the "big-endian" and "native-endian" DT properties

On chips strapped for BE, we'll need to use ioread32be/iowrite32be instead of
ioread32/iowrite32.

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