tegra: clean up board include hell

The prototypes used in board files were all scattered out, which lead to
code duplication between SPL and normal U-Boot and some prototypes not actually
being used. Consolidate this in a common board header.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>