spi: altera: add platform data for slave information.

This patch introduces platform data for slave information, it allows
spi-altera to add new spi devices once master registration is done.

Signed-off-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
Signed-off-by: Russ Weight <russell.h.weight@intel.com>
Reviewed-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/1591845911-10197-4-git-send-email-yilun.xu@intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>