Add comments describing the platform data members
and per-chip-select SPI configuration structure.

Also, add some comments describing the what happens
during the driver probe.
Signed-off-by: NBrian Niebuhr <bniebuhr@efjohnson.com>
Tested-By: NMichael Williamson <michael.williamson@criticallink.com>
Signed-off-by: NSekhar Nori <nsekhar@ti.com>