syscon: add Linux-compatible syscon API

syscon: add Linux-compatible syscon API

The syscon implementation in U-Boot is different from that in Linux.
Thus, DT files imported from Linux do not work for U-Boot.

In U-Boot driver model, each node is bound to a dedicated driver
that is the most compatible to it.  This design gets along with the
concept of DT, and the syscon in Linux originally worked like that.

However, Linux commit bdb0066df96e ("mfd: syscon: Decouple syscon
interface from platform devices") changed the behavior because it is
useful to let a device bind to another driver, but still work as a
syscon provider.

That change had happened before U-Boot initially supported the syscon
driver by commit 6f98b7504f70 ("dm: Add support for generic system
controllers (syscon)").  So, the U-Boot's syscon works differently
from the beginning.  I'd say this is mis-implementation given that
DT is not oriented to a particular project, but Linux is the canon
of DT in practice.

The problem typically arises in the combination of "syscon" and
"simple-mfd" compatibles.

In Linux, they are orthogonal, i.e., the order between "syscon" and
"simple-mfd" does not matter at all.

Assume the following compatible.

   compatible = "foo,bar-syscon", "syscon", "simple-mfd";

In U-Boot, this device node is bound to the syscon driver
(driver/core/syscon-uclass.c) since the "syscon" is found to be the
most compatible.  Then, syscon_get_regmap() succeeds.

However,

   compatible = "foo,bar-syscon", "simple-mfd", "syscon";

does not work because this node is bound to the simple-bus driver
(drivers/core/simple-bus.c) in favor of "simple-mfd" compatible.
The compatible string "syscon" is just dismissed.

Moreover,

   compatible = "foo,bar-syscon", "syscon";

works like the first case because the syscon driver populates the
child devices.  This is wrong because populating children is the job
of "simple-mfd" (or "simple-bus").

This commit ports syscon_node_to_regmap() from Linux.  This API
does not require the given node to be bound to a driver in any way.

Reported-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>