ARM: dts: stm32mp15: remove shmem for scmi-optee

Since OP-TEE commit 89ba3422ee80 ("plat-stm32mp1: scmi_server: default
use OP-TEE shared memory"), integrated in OP-TEE 3.22.0-rc1
the default configuration for STM32MP15x SoCs changes,
CFG_STM32MP1_SCMI_SHM_SYSRAM is disabled by default and the OP-TEE SMCI
server uses ithe OP-TEE native shared memory registered by clients.

To be compatible by default with this configuration and the next OP-TEE
version, this patch removes the SHMEM in the SCMI configuration and the
associated reserved memory in the last 4KByte page of SRAM,
in the STM32MP15 device tree.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>