ARM: Move kirkwood DT boards into mach-mvebu

Move the kirkwood DT support into mach-mvebu, and make them part of
ARCH_MULTI_V5. Minimal changes have been made in order to make it
boot. More intergration will take place in following patches.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
ARM: MVEBU: Let kirkwood use the system controller for restart

The mvebu system controller already supports restarting orion
systems. Make use of it, rather than the kirkwood specific code.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>