ARM: OMAP: mcbsp: Make threshold based transfer code generic

Remove CONFIG_ARCH_OMAP3 conditional compilation and cpu_is_omap34xx test
around buffer threshold based transfer and DMA operating mode control. Use
instead the buffer_size in platform data to determine when these sysfs
controls are exposed and when to access related McBSP registers. Rationale
for this is to make code generic and to allow to use it on OMAP4 that also
supports threshold based transfers.

Currently buffer_size variable is set only for OMAP3 SoCs but it is easy
to extend to OMAP4 and any later OMAP version.

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>