ARM: picoxcell: fix sched_clock() cleanup fallout

Commit 2f0778af (ARM: 7205/2: sched_clock: allow sched_clock to be
selected at runtime) replaced the picoxcell specific sched_clock() with
a generic runtime selectable version but replaced "unsigned long long
notrace sched_clock(void)" with "unsigned u32 notrace
picoxcell_read_sched_clock(void)" fix this up to return a u32 as
expected.

Cc: Marc Zyngier <Marc.Zyngier@arm.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>