xtensa: do not imply atomic ops kconfig

Xtensa's arch level atomic implementation requries S32C1I
support which may not exist as Xtensa cores are highly
configurable. Implying CONFIG_ATOMIC_OPERATIONS_ARCH at
arch level has a side effect that it is enabled
regardless if the other options are enabled. Given how
the header file's #ifdef is structured, only C atomic
ops can override this, which means enabling
CONFIG_ATOMIC_OPERATIONS_BUILTIN has no effect at all.
So let the SoC or board decide which atomic ops are
being used instead.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>