enable highmem for rpi2

target/linux/config/Config.in.kernel

@@ -244,6 +244,7 @@ config ADK_KERNEL_HIGHMEM
 	default y if ADK_TARGET_CUBOX_I2ULTRA
 	default y if ADK_TARGET_CUBOX_I2EX
 	default y if ADK_TARGET_SYSTEM_IBM_X40
+	default y if ADK_TARGET_SYSTEM_RASPBERRY_PI2
 	default n
 	help
 	  Enable high memory support.