dt-bindings: mailbox: nvidia,tegra186-hsp: Use generic node name

According to Devicetree specifications, device node names should be
generic, thus Mailbox provider should be called "mailbox", not "hsp".

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>