Add missing support for BL2_AT_EL3 in XIP memory

Add the missing flag for aarch32 XIP memory mode. It was
previously added in aarch64 only.
Minor: Correct the aarch64 missing flag.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: Iac0a7581a1fd580aececa75f97deb894858f776f