bus: brcmstb_gisb: Rework dependencies

Do not have the machine Kconfig entry point need to select
BRCMSTB_GISB_ARB, instead, just let it be default ARCH_BRCMSTB which is
a better way to deal with this. While at it, also make it default
BMIPS_GENERIC so the legacy MIPS-based STB platforms can benefit from
the same thing.

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