fsldma: Remove CONFIG_FSL_DMA_SELFTEST, keep fsl_dma_self_test() running always.

Always enabling the fsl_dma_self_test() to ensure the DMA controller
should works well after the driver probed.

Signed-off-by: Zhang Wei <wei.zhang@freescale.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>