dm: core: Add device checking to syscon_get_regmap()

dm: core: Add device checking to syscon_get_regmap()

This function can only handle a syscon device. It is possible that someone
will make a mistake, so add a check for this.

Also we should return -ENODEV when a device cannot be found, so update the
syscon_get_regmap_by_driver_data() to follow this convention.

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