devicetree: bindings: document Broadcom CPU enable method

Broadcom mobile SoCs use a ROM-implemented holding pen for
controlled boot of secondary cores.  A special register is
used to communicate to the ROM that a secondary core should
start executing kernel code.  This enable method is currently
used for members of the bcm281xx and bcm21664 SoC families.

The use of an enable method also allows the SMP operation vector to
be assigned as a result of device tree content for these SoCs.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>