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>