tegra: fdt: Add tegra186-u-boot.dtsi

All Tegra chips except Tegra186 have a tegraNNN-u-boot.dtsi. Duplicate
Tegra210's copy of this file for Tegra186. This ensures that a /binman node
exists in U-Boot's control DT. Subsequent to 3c10dc95 ("binman: Add a
library to access binman entries") this appears to be required. I haven't
really investigated why all this is necessary or how it works, but simply
observed the boot failure listed below, bisected it, noticed the
inconsistency in DT files, and found that fixing it resolved the boot
issue.

    U-Boot 2020.01-rc4-00256-g3c10dc95bdd0 (Jan 07 2020 - 10:25:00 -0700)

    SoC: tegra186
    Model: NVIDIA P2771-0000-500
    Board: NVIDIA P2771-0000
    DRAM:  7.8 GiB
    initcall sequence 00000000fffb7858 failed at call 00000000800955a8 (err=-22)
    ### ERROR ### Please RESET the board ###

Fixes: 3c10dc95 ("binman: Add a library to access binman entries")
Fixes: f2faffec ("binman: tegra: Convert to use binman")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>