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/Kconfig

@@ -911,6 +911,7 @@ config CAVIUM_OCTEON_SOC
 	select MIPS_NR_CPU_NR_MAP_1024
 	select BUILTIN_DTB
 	select MTD_COMPLEX_MAPPINGS
+	select SWIOTLB
 	select SYS_SUPPORTS_RELOCATABLE
 	help
 	  This option supports all of the Octeon reference boards from Cavium
@@ -1366,6 +1367,7 @@ config CPU_LOONGSON3
 	select MIPS_PGD_C0_CONTEXT
 	select MIPS_L1_CACHE_SHIFT_6
 	select GPIOLIB
+	select SWIOTLB
 	help
 		The Loongson 3 processor implements the MIPS64R2 instruction
 		set with many extensions.