ns9xxx: move registration of serial8250 to a dedicated file

Now the needed structs are allocated dynamically from __init code reducing
memory usage if the kernel runs on a board different from a9m9750dev.

Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
diff --git a/arch/arm/mach-ns9xxx/Makefile b/arch/arm/mach-ns9xxx/Makefile
index 6fb82b8..ca2c68d 100644
--- a/arch/arm/mach-ns9xxx/Makefile
+++ b/arch/arm/mach-ns9xxx/Makefile
@@ -5,3 +5,6 @@
 
 obj-$(CONFIG_BOARD_A9M9750DEV) += board-a9m9750dev.o
 obj-$(CONFIG_BOARD_JSCC9P9360) += board-jscc9p9360.o
+
+# platform devices
+obj-$(CONFIG_NS9XXX_HAVE_SERIAL8250) += plat-serial8250.o