spl: fit: Eanble GZIP support for image decompression

Add Kconfig option SPL_GZIP and SPL_ZLIB to enable gunzip support for
SPL boot, eg. falcon boot compressed kernel image.

Signed-off-by: York Sun <york.sun@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>