dt-bindings: mfd: ti,j721e-system-controller: Fix mux node errors

The ti,j721e-system-controller binding does not follow the standard mux
controller node name 'mux-controller' and the example is incomplete. Fix
these to avoid schema errors before the mux controller binding is
converted to schema.

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210526184839.2937899-2-robh@kernel.org