ASoC: samsung: i2s: Add spinlock in place of local_irq_* calls

It seems this driver hasn't been updated for SMP, as local_irq_save/
local_irq_restore don't provide proper protection of read/modify/write
of the device's registers on such systems. Introduce a spinlock
serializing access to the register region, it will be helpful later
when I2SMOD, I2SPSR registers are made also accessible through the
clk API.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>