Instead of using defines for the RAM size and base address, retrieve these
information from the devicetree.
This aligns with the sama5d27_som1_ek board in commit 73c1589f02
("board: sama5d27_som1_ek: Get dram size and base from device tree").
While at it, remove a spurious '/* SPL */' comment.
Signed-off-by: Fabio Estevam <festevam@denx.de>