ARM: ixp4xx: convert sched_clock() to use new infrastructure

Convert ixp4xx to use the new sched_clock() infrastructure for
extending 32bit counters to full 64-bit nanoseconds.

Tested-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>