spi: Make spi_alloc_device and spi_add_device public again

spi: Make spi_alloc_device and spi_add_device public again

This functions were previously made private since they
were not used. However, these functions will be needed
again.

Partial revert of commit fa70d3cd0ce8
("spi: Make several public functions private to spi.c")

Signed-off-by: Stefan Binding <sbinding@opensource.cirrus.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20220121172431.6876-2-sbinding@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>