sunxi: spl: Remove redundant check from `board_mmc_init` for signature

Remove the boot signature check from board_mmc_init() in spl mode, as it
is already done in spl_boot_device() in this case, and update the comments
to reflect this.

Signed-off-by: Daniel Kochmański <dkochmanski@turtle-solutions.eu>
CC: Roy Spliet <r.spliet@ultimaker.com>
Cc: Ian Campbell <ijc@hellion.org.uk>
[hdegoede@redhat.com: Disable the check only for SPL instead of always]
Acked-by: Hans De Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>