sandbox: Add a way to show the sandbox memory-mapping

This is mostly hidden in the background, but it is sometimes useful to
look at it. Add a function to allow this.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/sb.rst

@@ -12,6 +12,7 @@ Synopsis
 ::
 
     sb handoff
+    sb map
     sb state
 
 Description
@@ -26,6 +27,24 @@ sb handoff
 This shows information about any handoff information received from SPL. If
 U-Boot is started from an SPL build, it shows a valid magic number.
 
+sb map
+~~~~~~
+
+This shows any mappings between sandbox's emulated RAM and the underlying host
+address-space.
+
+Fields shown are:
+
+Addr
+    Address in emulated RAM
+
+Mapping
+    Equivalent address in the host address-space. While sandbox requests address
+    ``0x10000000`` from the OS, this is not always available.
+
+Refcnt
+    Shows the number of references to this mapping.
+
 sb state
 ~~~~~~~~
 
@@ -42,6 +61,12 @@ as ``sandbox_spl``::
     => sb handoff
     SPL handoff magic 14f93c7b
 
+This shows output from the *sb map* subcommand, with a single mapping::
+
+    Sandbox memory-mapping
+        Addr           Mapping  Refcnt
+    ff000000  000056185b46d6d0       2
+
 This shows output from the *sb state* subcommand::
 
     => sb state