spi/orion: Use module_platform_driver()

This patch reduces and simplifies initalization code by
using module_platform_driver().
With this change it's necessary to remove the __init annotation
to avoid section mismatch warnings.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>