ARM: dts: imx: Add devicetree for Kobo Aura 2

This adds a devicetree for the Kobo Aura 2 Ebook reader. It is based
on boards marked with "37NB-E60QL0+4B1". It is equipped with an i.MX6SL
SoC.

Expected to work:
  - Buttons
  - Wifi (with external module)
  - LED
  - uSD
  - USB
  - RTC
  - Fuel Gauge
  - Backlight (if the required regulator is probed before the backlight,
    specifying a supply is not supported by backlightdriver)

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>