swiotlb: move the SWIOTLB config symbol to lib/Kconfig

This way we have one central definition of it, and user can select it as
needed.  The new option is not user visible, which is the behavior
it had in most architectures, with a few notable exceptions:

 - On x86_64 and mips/loongson3 it used to be user selectable, but
   defaulted to y.  It now is unconditional, which seems like the right
   thing for 64-bit architectures without guaranteed availablity of
   IOMMUs.
 - on powerpc the symbol is user selectable and defaults to n, but
   many boards select it.  This change assumes no working setup
   required a manual selection, but if that turned out to be wrong
   we'll have to add another select statement or two for the respective
   boards.

Signed-off-by: Christoph Hellwig <hch@lst.de>

arch/mips/loongson64/Kconfig

@@ -130,13 +130,6 @@ config LOONGSON_UART_BASE
 	default y
 	depends on EARLY_PRINTK || SERIAL_8250
 
-config SWIOTLB
-	bool "Soft IOMMU Support for All-Memory DMA"
-	default y
-	depends on CPU_LOONGSON3
-	select DMA_DIRECT_OPS
-	select NEED_DMA_MAP_STATE
-
 config PHYS48_TO_HT40
 	bool
 	default y if CPU_LOONGSON3