tegra2: Simplify tegra_start() boot path

The Tegra2 boot path is more complicated than it needs to be. Since we want
to move to building most of U-Boot with ARMv7 and only a small part with
ARMv4T (for AVP) it should be as simple as possible.

This makes tegra2_start() into a simple function which either does AVP
init or A9 init depending on which core is running it. Both cores now
following the same init path, beginning at _start, and the special Tegra2
boot path code is no longer required.

Only two files need to be built for ARMv4T, and this is handled in the
Tegra2 CPU Makefile.

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