cmd: malloc: Add a command to show the malloc log

Add a command interface for the malloc-traffic log:
- malloc log start: Start recording allocations
- malloc log stop: Stop recording
- malloc log: Dump the recorded entries

Example output:

  => malloc log
  Malloc log: 29 entries (max 524288, total 29)
   Seq  Type                   Ptr      Size  Caller
  ----  --------  ----------------  --------  ------
     0  free              16a016e0         0  free_pipe_list:2001
                <-parse_stream_outer:3208 <-parse_file_outer:3300
     1  alloc             16a01b90        20  hush_file_init:3277
                <-parse_file_outer:3295 <-run_pipe_real:1986

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

doc/develop/malloc.rst

@@ -390,7 +390,8 @@ Malloc-Traffic Log
 
 On sandbox, when mcheck is enabled, a malloc-traffic log can record all
 malloc/free/realloc calls. This is useful for debugging allocation patterns
-and finding where allocations without caller info originate.
+and finding where allocations without caller info originate. See
+:doc:`../usage/cmd/malloc` for usage.
 
 The log buffer is allocated from host memory using ``os_malloc()``, so it
 does not affect U-Boot's heap. The size is controlled by