ASoC: fsi: Add spin lock operation for accessing shared area

fsi_master_xxx function should be protected by spin lock,
because it are used from both FSI-A and FSI-B.

Signed-off-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>