fdt: Add U-Boot version to chosen node

Add a new device tree property "u-boot,version" in the chosen node to
pass the U-Boot version to the operating system.
This can be useful to implement a firmware upgrade procedure from the
operating system.

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Tom Rini <trini@konsulko.com>