Revert "i2c: rcar: remove spinlock"

Revert "i2c: rcar: remove spinlock"

This reverts commit 150b8be3cda54412ad7b54f5392b513b25c0aaa7.

The I2C core's per-adapter locks can't protect from IRQs, so the driver still
needs a spinlock to protect the register accesses.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: stable@vger.kernel.org # 3.16+
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
(cherry picked from commit 91bfe2989af02e709ca01ccf518c4fbda3efc70f)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>