ARM: dts: imx: Add devicetree for Kobo Clara 2E

ARM: dts: imx: Add devicetree for Kobo Clara 2E

Adds a devicetree for the Kobo Clara 2E Ebook reader. It is based
on boards marked with "37NB-E60K2M+4A2" or "37NB-E60K2M+4B0". It is
equipped with an i.MX6SLL SoC.

Expected to work:
  - Buttons
  - Wifi
  - Bluetooth
    (if Wifi is initialized first, driver does not handle regulators
     yet)
  - LED
  - uSD
  - USB
  - RTC
  - Touchscreen

Add human-readable comments for devices without mainlined driver and
binding. Such comments can e.g. be help to find testers if someone
starts to work on the missing drivers.

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