ls1028a: use default scan_dev_for_boot

scan_dev_for_efi is supposed to be called from scan_dev_for_boot.
However this call is missing for ls1028ardb and ls1028aqds boards. As
a result EFI boot doesn’t work. Fix this issue by removing custom
definition of scan_dev_for_boot and use the default definition
instead.

Signed-off-by: Mian Yousaf Kaukab <ykaukab@suse.de>
Reviewed-by:  <andy.tang@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>