test: Add tests for the multiplexer framework

Provide tests to check the behavior of the multiplexer framework.

Two sets of tests are added. One is using an emulated multiplexer driver
that can be used to test basic functionality like select, deselect, etc.
The other is using the mmio mux which adds tests specific to it.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>