TrimSlice can be booted using device tree with equal functionality as
when booted using a board file. Remove the board file since it's no
longer needed.

One special-case is still left in board-dt-tegra20.c, since the Tegra
PCIe driver doesn't support device tree yet. This logic is now enabled
by CONFIG_TEGRA_PCI rather than via CONFIG_MACH_TRIMSLICE. The extra
cases where it's enabled (.configs which did not enable TrimSlice
support) shouldn't impact much since the amount of code is tiny.
Signed-off-by: Stephen Warren <swarren@nvidia.com>