dt-bindings: mfd: Correct the node name of the panel LED

According to the definition in leds-pwm.yaml, the node name of each LED
must match the regular expression "^led(-[0-9a-f]+)?$". "led" or "led-"
followed by a decimal or hexadecimal ID number.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>