spi/omap2_mcspi: add turbo mode support

Turbo mode allows to read data to shift register when rx-buffer
is full thus improving the perfomance. This feature is available
for RX-only mode.

In PIO turbo mode when the penultimate word is available
in RX-buffer the controller should be disabled before reading data
to prevent the next transaction triggering. The controller itself
handles the last word to be correctly loaded to shift-register and
then transferred to RX-buffer.

The turbo mode is enabled by setting turbo_mode parameter to 1.
This parameter is a part of omap2_mcspi_device_config structure
which is passed through the spi_device controller_data pointer.

Signed-off-by: Roman Tereshonkov <roman.tereshonkov@nokia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>