mux: mmio: Do not use syscon helper to build regmap

mux: mmio: Do not use syscon helper to build regmap

The syscon helper device_node_to_regmap() is used to fetch a regmap
registered to a device node. It also currently creates this regmap
if the node did not already have a regmap associated with it. This
should only be used on "syscon" nodes. This driver is not such a
device and instead uses device_node_to_regmap() on its own node as
a hacky way to create a regmap for itself.

This will not work going forward and so we should create our regmap
the normal way by defining our regmap_config, fetching our memory
resource, then using the normal regmap_init_mmio() function.

Signed-off-by: Andrew Davis <afd@ti.com>
Tested-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20250123182059.597491-1-afd@ti.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>