spi: Add function to allocate a new SPI slave

At present it is difficult to extend the SPI structure since all
drivers allocate it themselves, and few of them zero all fields. Add
a new function spi_alloc_slave() which can be used by SPI drivers
to perform this allocation, and thus ensure that all drivers can
better cope with SPI structure changes.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/spi/Makefile b/drivers/spi/Makefile
index b8264df..45862ec 100644
--- a/drivers/spi/Makefile
+++ b/drivers/spi/Makefile
@@ -25,6 +25,9 @@
 
 LIB	:= $(obj)libspi.o
 
+# There are many options which enable SPI, so make this library available
+COBJS-y += spi.o
+
 COBJS-$(CONFIG_ALTERA_SPI) += altera_spi.o
 COBJS-$(CONFIG_ANDES_SPI) += andes_spi.o
 COBJS-$(CONFIG_ARMADA100_SPI) += armada100_spi.o