mfd: Add DT bindings documentation for Samsung Exynos LPASS

This patch adds documentation of the DT bindings for the Samsung
Exynos SoC Low Power Audio Subsystem.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>