xtensa: allow arch-specific arch_spin_relax() with more NOPs

This adds a Kconfig to introduce the Xtensa specific
arch_spin_relax() which can do more NOPs. Some Xtensa SoCs
may need more NOPs after failure to lock a spinlock,
especially under SMP. This gives the bus extra time to
propagate the RCW transactions among CPUs.

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