board_f: sandbox: Move setup_ram_buf() to private code

There is no need to have this call in the generic init sequence and no
other architecture has needed it in the time it has been there. Move it
into sandbox's private code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>