spi/dw_spi: move dw_spi.h into drivers/spi

include/linux/dw_spi.h only includes driver internal data.  It doesn't
expose a platform_data configuration structure or similar (at least
nothing in-tree).  This patch moves the header into drivers/spi so
that the scope is limited to only the dw_spi_*.c driver files

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Feng Tang <feng.tang@intel.com>
Cc: spi-devel-general@lists.sourceforge.net