ARM: restrict BCM_KONA_UART to ARCH_BCM_MOBILE

Low-level debugging using the Broadcom Kona UART only makes sense on the
ARCH_BCM_MOBILE platform and would otherwise prevent ARCH_BCM_63XX from
picking up the right UART implementation by default.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>

arch/arm/Kconfig.debug

@@ -113,7 +113,7 @@ choice
 
 	config DEBUG_BCM_KONA_UART
 		bool "Kernel low-level debugging messages via BCM KONA UART"
-		depends on ARCH_BCM
+		depends on ARCH_BCM_MOBILE
 		select DEBUG_UART_8250
 		help
 		  Say Y here if you want kernel low-level debugging support