am33xx: Use regular spl_board_init instead of am33xx_spl_board_init

am33xx: Use regular spl_board_init instead of am33xx_spl_board_init

The am33xx_spl_board_init function was introduced as a way to add
board-specific SPL init for AM33xx devices since the spl_board_init
function was already used for SoC-specific init.

Now that the SoC-specific SPL init was moved to spl_soc_init, we can
use spl_board_init for this purpose and get rid of
am33xx_spl_board_init.

Rename the function in board files and enable the related config
option for concerned boards.

Signed-off-by: Paul Kocialkowski <paulk@sys-base.io>
Reviewed-by: Tom Rini <trini@konsulko.com>