Introduce arch_phys_memset which works like memset but on physical memory

The default implementation of this function is just memset, but other
implementations will be needed when physical memory isn't accessible by
U-Boot using normal addressing mechanisms.

Signed-off-by: Gabe Black <gabeblack@chromium.org>
Signed-off-by: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index ed7d270..c7aab18 100644
--- a/README
+++ b/README
@@ -2200,6 +2200,14 @@
 			HERMES, IP860, RPXlite, LWMON,
 			FLAGADM, TQM8260
 
+- Access to physical memory region (> 4GB)
+		Some basic support is provided for operations on memory not
+		normally accessible to U-Boot - e.g. some architectures
+		support access to more than 4GB of memory on 32-bit
+		machines using physical address extension or similar.
+		Define CONFIG_PHYSMEM to access this basic support, which
+		currently only supports clearing the memory.
+
 - Error Recovery:
 		CONFIG_PANIC_HANG