ARM: OMAP: mcbsp: Cleanup sidetone control initialization and make it generic

Sidetone resource is already registered for a device so there is no need
for cpu_is_omap34xx() and McBSP port number tests in the driver. We can
cleanup and make the code generic by dropping remaining CONFIG_ARCH_OMAP3
conditional compilations and then using sidetone resource and st_data
variable for runtime tests.

Signed-off-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Signed-off-by: Tony Lindgren <tony@atomide.com>