common: Make sure arch-specific map_sysmem() is defined

In the case where the arch defines a custom map_sysmem(), make sure that
including just mapmem.h is sufficient to have these functions as they
are when the arch does not override it.

Also split the non-arch specific functions out of common.h

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/lib/bootm.c b/arch/arm/lib/bootm.c
index 2d6b676..b1bff8c 100644
--- a/arch/arm/lib/bootm.c
+++ b/arch/arm/lib/bootm.c
@@ -18,6 +18,7 @@
 #include <u-boot/zlib.h>
 #include <asm/byteorder.h>
 #include <libfdt.h>
+#include <mapmem.h>
 #include <fdt_support.h>
 #include <asm/bootm.h>
 #include <asm/secure.h>