mfd: syscon: Remove the platform driver support

The platform driver is dead code. It is not used by DT platforms since
commit bdb0066df9 ("mfd: syscon: Decouple syscon interface from
platform devices") which said:

    For non-DT based platforms, this patch keeps syscon platform driver
    structure so that syscon can be probed and such non-DT based drivers
    can use syscon_regmap_lookup_by_pdev API and access regmap handles.
    Once all users of "syscon_regmap_lookup_by_pdev" migrated to DT based,
    we can completely remove platform driver of syscon, and keep only helper
    functions to get regmap handles.

The last user of syscon_regmap_lookup_by_pdevname() was removed in 2018.
syscon_regmap_lookup_by_pdevname() was then removed in 2019, but that
commit failed to remove the rest of the platform driver.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Tested-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Tested-by: Will McVicker <willmcvicker@google.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Tested-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20241217-syscon-fixes-v2-2-4f56d750541d@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>

include/linux/platform_data/syscon.h

@@ -1,9 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0 */
-#ifndef PLATFORM_DATA_SYSCON_H
-#define PLATFORM_DATA_SYSCON_H
-
-struct syscon_platform_data {
-	const char *label;
-};
-
-#endif