arm: orion5x: basic Device Tree support

This commit adds basic DT support for the Orion5x SoC family. It adds
an orion5x.dtsi description of the Orion5x SoC as well as the needed
DT_MACHINE structure to support boards converted to DT in the future.

So far, the Device Tree contains the interrupt controller, the GPIO
bank, the UART controllers, the SPI controller, the watchdog, the SATA
controller, the I2C controller and the cryptographic engine.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested by: Maxime Hadjinlian <mhadjinlian@lacie.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>