ARM: dts: microchip: at91: align LED node name with bindings

ARM: dts: microchip: at91: align LED node name with bindings

Bindings expect the LED node names to follow certain pattern, see
dtbs_check warnings:

  at91sam9g15ek.dtb: leds: 'pb18', 'pd21' do not match any of the regexes: '(^led-[0-9a-f]$|led)', 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alexander Dahl <ada@thorsis.com>
Link: https://lore.kernel.org/r/20240701164952.577277-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>