ppc: xilinx-ppc4xx: Port to DM serial

xilinx_uartlite has been ported to DM, this patch makes the
xilinx-ppc405-generic and the xilinx-ppc440-generic boards use the new
DM driver.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/board/xilinx/ppc405-generic/xilinx_ppc405_generic.c b/board/xilinx/ppc405-generic/xilinx_ppc405_generic.c
index 8b10dba..32105a8 100644
--- a/board/xilinx/ppc405-generic/xilinx_ppc405_generic.c
+++ b/board/xilinx/ppc405-generic/xilinx_ppc405_generic.c
@@ -46,3 +46,7 @@
 	return;
 }
 void get_sys_info(sys_info_t *) __attribute__((weak, alias("__get_sys_info")));
+
+int get_serial_clock(void){
+	return XPAR_UARTNS550_0_CLOCK_FREQ_HZ;
+}