ARM: OMAP: mcbsp: Use per instance register cache size

Rationale here is to remove one global variable and to make possible to have
variable size McBSP register maps inside SoC.

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>