orion5x: allow overriding default mappings windows

Turn all ORION5X_DEF{ADR,SZ}_xxx macros into ORION5X_{ADR,SZ}_xxx
and allow defining them from board code to override defaults. This
is particularly useful for defining board-specific FLASH address
and size in board header file rather than having to tweak orion5x
code.

Signed-off-by: Albert Aribaud <albert.aribaud@free.fr>