Convert CONFIG_PHYSMEM to Kconfig

This converts the following to Kconfig:
   CONFIG_PHYSMEM

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Sean Anderson <seanga2@gmail.com>
diff --git a/lib/Kconfig b/lib/Kconfig
index 807a4c6..10ba086 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -15,6 +15,16 @@
 	help
 	  Sets the number of entries in the virtual-physical mapping table.
 
+config PHYSMEM
+	bool "Access to physical memory region (> 4G)"
+	help
+	  Some basic support is provided for operations on memory not
+	  normally accessible to 32-bit U-Boot - e.g. some architectures
+	  support access to more than 4G of memory on 32-bit
+	  machines using physical address extension or similar.
+	  Enable this to access this basic support, which only supports clearing
+	  the memory.
+
 config BCH
 	bool "Enable Software based BCH ECC"
 	help