spi: altera: separate core code from platform code

In preparation of adding support for a new bus type,
separate the core spi-altera code from the platform
driver code.

Signed-off-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
Link: https://lore.kernel.org/r/20210416165720.554144-2-matthew.gerlach@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>