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 already did for the
current zlib (0.95). New zlib guarantees a faster inflate performances
other then others improvements as explained at www.zlib.net.
It also includes Alessandro Rubini's patches to allow 0 as destination pointer
and to call watchdog reset if required by architecture.

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