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

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 <[email protected]>
CC: Roy Spliet <[email protected]>
Cc: Ian Campbell <[email protected]>
[[email protected]: Disable the check only for SPL instead of always]
Acked-by: Hans De Goede <[email protected]>
Signed-off-by: Hans de Goede <[email protected]>