fdts: stm32mp15_bl2: add the trusted boot configuration

Add the trusted boot firmware configuration information into
the BL2 device tree. It enabled the authentication.
MBEDTLS heap is not defined, it will use the hard coded one.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Change-Id: I47181d179b814dc512dc527ae54a5a19a126f1c5
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/184833


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>