efi: Avoid using dm_scan_other()

This function is defined by bootstd so using it precludes using that
feature. Use the board_early_init_r() feature instead.

Nove the affected code into a new file, so that it is clear that
this relates to board init.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>