dt-bindings: microchip: atmel,at91rm9200-tcb: Add atmel,tcb-pwm

Move the TCB pwm nodes under their parent. This removes the need for the
tc-block property as there is now a child-parent relationship between the
TC channel and the TC block.

Move the documentation to the main file.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>