ARM: defconfig: orion5x: add DT support

- add 2 DT device entries
- enable CONFIG_ARM_ATAG_DTB_COMPAT

Signed-off-by: Roger Shimizu <rogershimizu@gmail.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>