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: NGiuseppe Condorelli <giuseppe.condorelli@st.com>
Reviewed-by: NAngelo Castello <angelo.castello@st.com>

Edited commit message
Signed-off-by: NWolfgang Denk <wd@denx.de>