Linux also provides a device_node_to_regmap function that doesn't do a
compatible = "syscon" check. In preparation for having it in barebox,
factor out the compatible check into the callers.

While at it make the clock checking explicit, so the diff clearly
indicates, we touched all instances and to prepare for follow-up
commits.

No functional change.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>