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>

board/menlo/m53menlo/m53menlo.c

@@ -20,6 +20,7 @@
 #include <asm/spl.h>
 #include <fdt_support.h>
 #include <fsl_esdhc_imx.h>
+#include <gzip.h>
 #include <i2c.h>
 #include <ipu_pixfmt.h>
 #include <linux/errno.h>