xtensa: select generic atomic64_t support

This will fix build errors:

block/blk-cgroup.c:609:2: error: unknown type name 'atomic64_t'
block/blk-cgroup.c:609:2: error: implicit declaration of function 'ATOMIC64_INIT' [-Werror=implicit-function-declaration]

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/xtensa/Kconfig

@@ -7,6 +7,7 @@ config ZONE_DMA
 config XTENSA
 	def_bool y
 	select HAVE_IDE
+	select GENERIC_ATOMIC64
 	select HAVE_GENERIC_HARDIRQS
 	select GENERIC_IRQ_SHOW
 	select GENERIC_CPU_DEVICES