ARM/dts: omap3: Fix mcbsp2/3 hwmods to be able to probe the drivers for audio

Fixes the following errors:
[    2.318084] omap-mcbsp 49022000.mcbsp: invalid rx DMA channel
[    2.324432] omap-mcbsp 49024000.mcbsp: invalid rx DMA channel

Which is because we failed to link the sidetone hwmod for McBSP2/3. The
missing sidetone hwmod link will prevent omap_device_alloc() to append the
DMA resources since we - accidentally - end up having the same number of
resources provided from DT (IO/IRQ) as we have in hwmod for the McBSP ports
without the ST resources.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>