ARM: tegra: add SoC-level hook for board_late_init()

ARM: tegra: add SoC-level hook for board_late_init()

Extend the Tegra186 implementation of board_late_init() to call a per-SoC
"hook" function. This will allow SoC-specific (rather than Tegra-wide)
functionality to be implemented without the core Tegra code needing to be
aware of the details. While board186.c is currently only used for
Tegra186, it should be applicable to any other future SoC, and perhaps its
simple design could be back-ported to older SoCs in the future too.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>