imx: spl: Partially revert "spl: eMMC/SD: Provide one __weak spl_boot_mode() function"

Commit d695d662 ("spl: eMMC/SD: Provide one __weak spl_boot_mode()
function") breaks the boot on several i.MX6 boards,
such as cuboxi and wandboard:

U-Boot SPL 2018.03-rc1-00212-g48914fc119 (Feb 10 2018 - 11:04:33 +1300)
Trying to boot from MMC1
Failed to mount ext2 filesystem...
spl_load_image_ext: ext4fs mount err - 0

Partially revert it so that we can boot U-Boot again on these
i.MX6 platforms.

Reported-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Breno Lima <breno.lima@nxp.com>