OMAP: McBSP: Create and export max_(r|t)x_thres property

This patch export through sysfs two properties to configure
maximum threshold for transmission and reception on each
mcbsp instance. Also, it exports two helper functions to
allow mcbsp users to read this values.

Signed-off-by: Eduardo Valentin <eduardo.valentin@nokia.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>