spi: Add function to allocate a new SPI slave

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>