ARM: at91: Document new TCB bindings

The current binding for the TCB is not flexible enough for some use cases
and prevents proper utilization of all the channels.

Acked-by: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>