ARM: dts: mmp3-dell-ariel: Add the embedded controller

Add the device node for the computer's embedded controller, responsible
for controlling the LEDs and system power.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20210121034130.1381872-10-lkundrak@v3.sk

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>