dts: bindings: dma: raspberrypi: Fix irq0-channels definition

`irq0-channels` defines even-numbered channels as the default value.
Since 6 was dropped from this definition, it is added.
Also, since the maximum number of channels is 12,
remove the ones that are exceeded.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>