sunxi: refactor serial base addresses to avoid asm/arch/cpu.h

At the moment we have each SoC's memory map defined in its own cpu.h,
which is included in include/configs/sunxi_common.h. This will be a
problem with the introduction of Allwinner RISC-V support.

Remove the inclusion of that header file from the common config header,
instead move the required serial base addresses (for the SPL) into a
separate header file. Then include the original cpu.h file only where
we really need it, which is only under arch/arm now.

This disentangles the architecture specific header files from the
generic code.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/arch/arm/mach-sunxi/gtbus_sun9i.c b/arch/arm/mach-sunxi/gtbus_sun9i.c
index cf011c4..5624621 100644
--- a/arch/arm/mach-sunxi/gtbus_sun9i.c
+++ b/arch/arm/mach-sunxi/gtbus_sun9i.c
@@ -8,6 +8,7 @@
 
 #include <common.h>
 #include <asm/io.h>
+#include <asm/arch/cpu.h>
 #include <asm/arch/gtbus_sun9i.h>
 #include <asm/arch/sys_proto.h>