arm: don't build swiotlb by default

swiotlb is only used as a library of helper for xen-swiotlb if Xen support
is enabled on arm, so don't build it by default.
Signed-off-by: NChristoph Hellwig <hch@lst.de>

arch/arm/Kconfig

@@ -1773,7 +1773,7 @@ config SECCOMP
 	  defined by each seccomp mode.
 
 config SWIOTLB
-	def_bool y
+	bool
 
 config PARAVIRT
 	bool "Enable paravirtualization code"
@@ -1806,6 +1806,7 @@ config XEN
 	depends on MMU
 	select ARCH_DMA_ADDR_T_64BIT
 	select ARM_PSCI
+	select SWIOTLB
 	select SWIOTLB_XEN
 	select PARAVIRT
 	help