Add a better sched_clock() to the ixp4xx platform,
implemented via its clocksource support.

This is based on the sched_clock() I implemented for
the IOP platform. Tested on a ds101 ixp420 machine.
Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>