ARM: dts: imx50: Add Kobo Aura DTS

The Kobo Aura is an e-book reader released in 2013.

With the devicetree in its current state, the kernel will boot and run
for about ten seconds. To solve this, the embedded controller needs to
be told that the system should stay powered on. This will be done in a
later patchset.

- The IOMUXC mode bits for the SD interfaces were taken from the
  vendor's U-Boot fork.
- The bus width of the eMMC is 4 bits in the vendor kernel, but I
  achieved better performance with 8 bits.
- The SDIO clock frequency for the WiFi chip is 25MHz in the vendor
  kernel, but the WiFi chip (BCM43362) supports 50MHz, which works
  reliably on this board and gives slightly better performance.
- The I2C pins' IOMUXC settings come from the vendor's U-Boot fork.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>