ARM i.MX25: Add devicetree

This adds a i.MX25 dtsi file along with the i.MX25 clock tree
documentation. The devicetree should be fairly complete for:

- uart
- fec
- i2c
- spi
- pwm
- nand
- gpio
- wdog
- esdhc
- flexcan

The more exotic devices currently miss clock bindings.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>