led: gpio: Check device compatible string to determine the top level node

Since 2d1deaf8 ("led: gpio: Drop duplicate OF "label" property parsing"),
all LED nodes have some sort of label. Use device_is_compatible(..."leds-gpio")
to determine whether this is a top-level node, since it is only the top
level node which is compatible with "leds-gpio", the GPIO LEDs subnodes
are not.

Fixes: 2d1deaf8 ("led: gpio: Drop duplicate OF "label" property parsing")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Sean Anderson <seanga2@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Steven Lawrance <steven.lawrance@softathome.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>