stm32mp: update the mmu configuration for SPL and prereloc

Overidde the weak function dram_bank_mmu_setup() to set the DDR
(preloc case) or the SYSRAM (in SPL case) executable before to enable
the MMU and configure DACR.

This weak function is called in dcache_enable/mmu_setup.

This patchs avoids a permission access issue when the DDR is marked
executable (by calling mmu_set_region_dcache_behaviour with
DCACHE_DEFAULT_OPTION) after MMU setup and domain access permission
activation with DACR in dcache_enable.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>