T210: Add support for 64-bit T210-based P2571 board

Based on Venice2, incorporates Stephen Warren's
latest P2571 pinmux table.

With Thierry Reding's 64-bit build fixes, this
will build and and boot in 64-bit on my P2571
(when used with a 32-bit AVP loader).

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