socfpga: fix the serial console on DE1-SoC

Previously, the TX LED would flash but nothing would appear on the
serial port, and the board would appear dead with a build of the
socfpga_cyclone5_defconfig. I have verified that adding the frequency to
the uart will fix the serial console on my board.

Thanks to @ehoffman on the Rocketboards forum:
https://forum.rocketboards.org/t/cyclonev-programming-fpga-from-u-boot/2230/30

Signed-off-by: Jade Lovelace <lists@jade.fyi>
Reviewed-by: Marek Vasut <marex@denx.de>

arch/arm/dts/socfpga_cyclone5_de1_soc.dts

@@ -75,6 +75,7 @@
 };
 
 &uart0 {
+	clock-frequency = <100000000>;
 	u-boot,dm-pre-reloc;
 };