dt-binding: serial: tegra: add new chips

Add new compatible string for Tegra186. It differs from earlier chips
as it has FIFO mode enable check and 8 byte DMA buffer.
Add new compatible string for Tegra194. Tegra194 has different error
tolerance levels for baud rate compared to older chips.
Signed-off-by: Krishna Yarlagadda <kyarlagadda@nvidia.com>
Link: https://lore.kernel.org/r/1567572187-29820-6-git-send-email-kyarlagadda@nvidia.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/nvidia,tegra20-hsuart.txt

 NVIDIA Tegra20/Tegra30 high speed (DMA based) UART controller driver.
 
 Required properties:
-- compatible : should be "nvidia,tegra30-hsuart", "nvidia,tegra20-hsuart".
+- compatible : should be,
+  "nvidia,tegra20-hsuart" for Tegra20,
+  "nvidia,tegra30-hsuart" for Tegra30,
+  "nvidia,tegra186-hsuart" for Tegra186,
+  "nvidia,tegra194-hsuart" for Tegra194.
+
 - reg: Should contain UART controller registers location and length.
 - interrupts: Should contain UART controller interrupts.
 - clocks: Must contain one entry, for the module clock.