spi/spi_s3c64xx: Move to subsys_initcall()

Allow the use of the S3C64xx SPI controller with things like PMICs by
moving the init up to subsys_initcall().

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jassi Brar <jassi.brar@samsung.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>