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>
diff --git a/configs/am335x_guardian_defconfig b/configs/am335x_guardian_defconfig
index 6b40d39..7c7041c 100644
--- a/configs/am335x_guardian_defconfig
+++ b/configs/am335x_guardian_defconfig
@@ -29,6 +29,7 @@
 CONFIG_AUTOBOOT_STOP_STR=" "
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_ARCH_MISC_INIT=y
+CONFIG_SPL_BOARD_INIT=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
 CONFIG_SPL_SYS_MALLOC=y
 CONFIG_SPL_SYS_MALLOC_SIZE=0x800000