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>
diff --git a/drivers/bus/Kconfig b/drivers/bus/Kconfig
index d4a3a31..8807495 100644
--- a/drivers/bus/Kconfig
+++ b/drivers/bus/Kconfig
@@ -58,6 +58,7 @@
 config BRCMSTB_GISB_ARB
 	bool "Broadcom STB GISB bus arbiter"
 	depends on ARM || MIPS
+	default ARCH_BRCMSTB || BMIPS_GENERIC
 	help
 	  Driver for the Broadcom Set Top Box System-on-a-chip internal bus
 	  arbiter. This driver provides timeout and target abort error handling