arm/tegra: Add device-tree support for TrimSlice board

* Add device-tree file for TrimSlice
* Add that to the list of .dts files to build
* Update board-dt.c to recognize TrimSlice board name

v2: Makefile: Add board-trimslice-pinmux.c to obj-$(CONFIG_MACH_TEGRA_DT).
v3: Makefile: Use brackets not braces around var names

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>