arm64: Add Juno board device tree.

This adds support for ARM's Juno development board (rev 0).
It enables most of the board peripherals: UART, I2C, USB, MMC and
100Mb ethernet. There is no support at the moment for clock setting
and HDLCD driver which depends on it.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>