docs: add TF-A version numbering information

Add a chapter "Version numbering" in release information file that
explains macros used for TF-A version. It also introduces VERSION_PATCH
macro that is used for LTS releases. A comment for this macro is also
added in Makefile.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I946b6cb91bb8454131f07b24534d28ab1aef1771
2 files changed