xtensa: enable XTENSA_HAL at SoC level

This moves enabling XTENSA_HAL to the SoC definitions.
As Xtensa SoCs are highly configurable, it is possible
that the generic Xtensa HAL provided in the tree is
not suitable. So only enable XTENSA_HAL only if
the generic version can be used.

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