spi/dw_spi: enable platform specific chipselect.

The driver core allows for a platform-specific chipselect assert/deassert
function, however the chipselect function in the core doesn't take advantage
of this fact.

This enables the use of a custom function, should it be defined.

Signed-off-by: George Shore <george@georgeshore.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>