docs: deprecate io_dummy driver

This drivers was only used by one upstream platform: STM32MP1 but only
when enabling the flag STM32MP_USE_STM32IMAGE. This flag and the
corresponding code is now removed from TF-A. The driver can then be
set to deprecated. It will be removed after v2.9 tag.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ib8242a7291c7011d7f96a6a83609ec1996dce520