zlib: updated to v.1.2.3

This patch updates zlib to the latest stable version.

Only relevant zlib parts were ported to u-boot tree, as was done for
the previously used version of zlib (0.95). New zlib gives faster
inflate performance and other improvements, see www.zlib.net

Signed-off-by: Giuseppe Condorelli <giuseppe.condorelli@st.com>
Reviewed-by: Angelo Castello <angelo.castello@st.com>

Edited commit message

Signed-off-by: Wolfgang Denk <wd@denx.de>