spi/spi_s3c64xx: Make probe more robust against missing board config

The S3C64xx SPI driver requires the machine to call s3c64xx_spi_set_info()
to select a few options, including the clock to use for the SPI controller.
If this is not done then a NULL will be passed as the clock name for
clk_get(), causing an obscure crash. Guard against this and other missing
configuration by validating that the clock name has been filled in in
the platform data that ets passed in.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>