board: bsh: imx8mn_bsh_smm_s2/s2pro: let clock system enable UART clock

Now that the UART driver can enable the required clocks, remove
the hard-coded clock enable.

Co-developed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>

board/bsh/imx8mn_smm_s2/spl.c

@@ -43,8 +43,6 @@ void spl_board_init(void)
 
 int board_early_init_f(void)
 {
-	init_uart_clk(3);
-
 	if (IS_ENABLED(CONFIG_NAND_MXS)) {
 		init_nand_clk();
 	}