powerpc: Remove device_type = "board_control" properties in .dts files

We don't want to encourage the bogus device_type usage.

The device type isn't used in the code, so we can simply remove it from
the documentation and dts files.

Boards should specify proper compatible entries instead.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>