lpuart: add a get_lpuart_clk function

It's not always true that LPUART clock is CONFIG_SYS_CLK_FREQ. This
patch provides a weak function get_lpuart_clk(), so that the clock
can be ovreridden on a specific board which uses different clock
for LPUART.

Signed-off-by: Shaohui Xie <Shaohui.Xie@nxp.com>
[YS: Reformat commit message]
Reviewed-by: York Sun <york.sun@nxp.com>

drivers/serial/serial_lpuart.c

@@ -170,9 +170,14 @@ static int lpuart_serial_probe(struct udevice *dev)
 }
 #else
 
+u32 __weak get_lpuart_clk(void)
+{
+	return CONFIG_SYS_CLK_FREQ;
+}
+
 static void _lpuart32_serial_setbrg(struct lpuart_fsl *base, int baudrate)
 {
-	u32 clk = CONFIG_SYS_CLK_FREQ;
+	u32 clk = get_lpuart_clk();
 	u32 sbr;
 
 	sbr = (clk / (16 * baudrate));