clk: at91: add I2S clock mux driver

This driver is a simple muxing driver that controls the
I2S's clock input by using syscon/regmap to change the parent.
The available inputs can be peripheral clock and generated clock.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
[sboyd@kernel.org: Fix SPDX tag comment style]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>