MIPS: BMIPS: Fix gisb-arb compatible string for 7435

MIPS: BMIPS: Fix gisb-arb compatible string for 7435

The SUN GISB arbiter was added with the wrong compatible string, leading to
using the wrong register layout, use the correct compatible string for this
chip: brcm,bcm7435-gisb-arb.

Fixes: 8394968be4c7 ("MIPS: BMIPS: Add BCM7435 dtsi")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: blogic@openwrt.org
Cc: cernekee@gmail.com
Cc: jogo@openwrt.org
Cc: jaedon.shin@gmail.com
Cc: pgynther@google.com
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12285/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>