stm32mp1: increase device tree size to 28KB

Increase the max device tree size to 28KB to build it
with the TRUSTED_BOARD_BOOT defined nodes.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I0b48f9085a6ad9c1494eb162de3ad17c124fd8fb
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/188765


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