ARM: tegra: prototype pinmux_init() in board.h

pinmux_init() is a board-level function, not a pinmux driver function.
Move the prototype to a board header rather than the driver header.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>