Provide a new function which can cope with obtaining information from
of-platdata instead of the device tree.

Signed-off-by: Simon Glass <sjg@chromium.org>

We have a way to find a regmap by its syscon driver data value. Add the same
for syscon itself.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

Many SoCs have a number of system controllers which are dealt with as a
group by a single driver. It is a pain to have to add lots of compatible
strings and/or separate drivers for each. Instead we can identify the
controllers by a number and request the address of the one we want.

Add a simple implementation of this which can be used by SoC driver code.

Signed-off-by: Simon Glass <sjg@chromium.org>