ARM: tegra: move tegra-ahb.h out of arch/arm/mach-tegra/

We wish to empty arch/arm/mach-tegra/include/mach/ as much as possible
to enable single zImage. Move tegra-ahb.h to a more central location
(suggested by Arnd, OK'd by Greg KH), and actually make tegra-ahb.c
include the header to ensure client and provider agree on the prototype.

Signed-off-by: Stephen Warren <swarren@nvidia.com>