feat(stm32mp15): manage OP-TEE shared memory

On STM32MP15, there is currently an OP-TEE shared memory area at the end
of the DDR. But this area will in term be removed. To allow a smooth
transition, a new flag is added (STM32MP15_OPTEE_RSV_SHM). It reflects
the OP-TEE flag: CFG_CORE_RESERVED_SHM. The flag is enabled by default
(no behavior change). It will be set to 0 when OP-TEE is aligned, and
then later be removed.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I91146cd8a26a24be22143c212362294c1e880264
4 files changed