serial: tegra20-hsuart: Fix typo in dmas DT binding description

The description for dmas references a second property, which should be
"dma-names" instead of "clock-names".

Signed-off-by: Mirza Krak <mirza.krak@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>