ARM: tegra: split p2771-0000 build

There are multiple versions of p2771-0000 board. There are SW visible
incompatible differences between the versions, and they are relevant to
U-Boot. Create separate "A02" and "B00" defconfigs (named after the first
and/or only board rev the defconfig supports) so that users can select
which build they want.

With the minimal set of HW currently enabled in U-Boot, the differences
are irrelevant, hence the DT files aren't different. However, that will
change in a future patch.

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