atheros: v3.18: relocate PCI host DMA base definition

Put AR2315_PCI_HOST_SDRAM_BASEADDR macro to DMA header, since this is
arbitrary value and not some hw specific constant. Also this relocation
decouples dma from HW specific header.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>

SVN-Revision: 44716