spi/s3c64xx: Use bus clocks created using clkdev

This patch modifies the driver to stop depending on the
clock names being passed from platform and switch over
to lookup clocks generic names using clkdev

Signed-off-by: Padmavathi Venna <padma.v@samsung.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>