common: Move gzip functions into a new gzip header

As part of the effort to remove things from common.h, create a new header
for the gzip functions. Move the function declarations to it and add
missing documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>

cmd/bmp.c

@@ -9,16 +9,16 @@
  */
 
 #include <common.h>
-#include <dm.h>
-#include <lcd.h>
-#include <mapmem.h>
 #include <bmp_layout.h>
 #include <command.h>
-#include <asm/byteorder.h>
+#include <dm.h>
+#include <gzip.h>
+#include <lcd.h>
 #include <malloc.h>
 #include <mapmem.h>
 #include <splash.h>
 #include <video.h>
+#include <asm/byteorder.h>
 
 static int bmp_info (ulong addr);