stm32mp: update MMU config before the relocation

Mark the top of ram, used for relocated U-Boot as a normal memory
(cacheable and executable) to avoid permission access issue when
U-Boot jumps to this relocated code.

When MMU is activated in pre-reloc stage; only the beginning of
DDR is marked executable.

This patch avoids access issue when DACR is correctly managed.

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