dts: arm: socfpga: fix socfpga_de10_nano console

Booting this board failed as the initial console isn't found since
commit c402e817 ("dts: arm: socfpga: merge gen5 devicetrees from linux")

The uart0 devicetree entry was missing "clock-frequency = <100000000>:"
since that commit

Fixes: c402e817 ("dts: arm: socfpga: merge gen5 devicetrees from linux")
Reported-by: rafael mello <rafaelmello_3@hotmail.com>
Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>