AP3: hwmod: add dev_attr for McBSP sidetone

Since the sidetone block is tightly coupled to the mcbsp, sidetone information
is directly added to mcbsp2 & 3 hwmod dev_attr.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
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