configs: stm32mp1: reduce DDR_CACHEABLE_SIZE to supported 256MB DDR

Reduces the CONFIG_DDR_CACHEABLE_SIZE, the size of DDR mapped cacheable
before relocation, to support DDR with only 256MB because the OP-TEE
reserved memory is located at end of the DDR.

By default the new size of 128MB cacheable memory is enough
in dram_bank_mmu_setup() for early_enable_caches() in arch_cpu_init()
and is correct for DDR size = 256MB.

After relocation the real size of DDR, excluding the no-map reserved
memory, is used after the U-Boot device tree parsing.

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