Merged in masayuki2009/nuttx.nuttx/irq_spinlock (pull request #550)

SMP: Introduce spin_lock_irqsave() and spin_unlock_irqrestore()

These APIs are simplified version of enter_critical_section() and
leave_critical_section() to protect data (e.g. registers) in SMP mode.
By using these APIs inside drivers, performace will be improved.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

Approved-by: Gregory Nutt <gnutt@nuttx.org>