mtd: nand: lpc32xx slc: disable DMA support in SPL builds

Testing and analysis shows that at the moment LPC32xx NAND SLC driver
can not get PL080 DMA backbone support in SPL build, because SPL NAND
loaders operate with subpage (ECC step to be precisely) reads, and
this is not supported in the NAND SLC + DMA + hardware ECC calculation
bundle.

The change removes a cautious build time warning and explicitly
disables DMA flavour of the driver for SPL builds, to reduce the
amound of #ifdef sections the code blocks are minimally reorganized.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>