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/board/tcl/sl50/board.c b/board/tcl/sl50/board.c
index 484b4e2..6c60c70 100644
--- a/board/tcl/sl50/board.c
+++ b/board/tcl/sl50/board.c
@@ -92,7 +92,7 @@
 const struct dpll_params dpll_ddr_sl50 = {
 		400, OSC-1, 1, -1, -1, -1, -1};
 
-void am33xx_spl_board_init(void)
+void spl_board_init(void)
 {
 	int mpu_vdd;