ARM: restart: ixp2000: use new restart hook

Hook these platforms restart code into the new restart hook rather
than using arch_reset().

In doing so, we split out the IXDP2401, IXDP2801 and IXDP2805 platform
specific restart code into their own platform files.

Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>