mfd: syscon: Restore device_node_to_regmap() for non-syscon nodes

Commit ba5095eb ("mfd: syscon: Allow syscon nodes without a
"syscon" compatible") broke drivers which call device_node_to_regmap()
on nodes without a "syscon" compatible. Restore the prior behavior for
device_node_to_regmap().

This also makes using device_node_to_regmap() incompatible with
of_syscon_register_regmap() again, so add kerneldoc for
device_node_to_regmap() and syscon_node_to_regmap() to make it clear
how and when each one should be used.

Fixes: ba5095eb ("mfd: syscon: Allow syscon nodes without a "syscon" compatible")
Reported-by: Vaishnav Achath <vaishnav.a@ti.com>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: Nishanth Menon <nm@ti.com>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Tested-by: Dhruva Gole <d-gole@ti.com>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20250124191644.2309790-1-robh@kernel.org


Signed-off-by: Lee Jones <lee@kernel.org>