ARM: orion5x: multiplatform support

The orion5x platform is now ready to be enabled for multiplatform
support, this patch does the switch over by modifying the Kconfig file,
the defconfig and removing the last mach/*.h header that becomes obsolete
with this.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>