serial: 8250_tegra: Add explicit include for of.h

With linux/acpi.h no longer implicitly including of.h, add an explicit
include of of.h to fix the following error:

drivers/tty/serial/8250/8250_tegra.c:68:15: error: implicit declaration of function 'of_alias_get_id' [-Werror=implicit-function-declaration]

Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>