spi: spi-s3c64xx: Use module_platform_driver()

subsys_init_call() initializes driver too early.
It's preventing to move DMA channel allocation at the begining
(driver probe).

This patch reduces and simplifies initalization code by
using module_platform_driver() macro.
It's also efficiently delaying driver startup.

Signed-off-by: Lukasz Czerwinski <l.czerwinski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>