next_striped_spin_lock_num() function

Return a spin lock number from the _striped_ range Returns a spin lock number in the range PICO_SPINLOCK_ID_STRIPED_FIRST to PICO_SPINLOCK_ID_STRIPED_LAST in a round robin fashion. This does not grant the caller exclusive access to the spin lock, so the caller must: -# Abide (with other callers) by the contract of only holding this spin lock briefly (and with IRQs disabled - the default via spin_lock_blocking()), and not whilst holding other spin locks. -# Be OK with any contention caused by the - brief due to the above requirement - contention with other possible users of the spin lock.