spi/dw_spi: Fix missing header

Currently, build on PPC dies with:
In file included from drivers/spi/dw_spi_mmio.c:16:
include/linux/spi/dw_spi.h:147: error: field ‘tx_sgl’ has incomplete type
include/linux/spi/dw_spi.h:149: error: field ‘rx_sgl’ has incomplete type

Add linux/scatterlist.h include to dw_spi.h, because we need to know
the contents of the structure.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>