ARM: integrator: register sched_clock directly

The detour through plat-versatile/sched-clock.c is hard to migrate
to multiplatform set-up and it's very little code being duplicated
so let's just inline the sched_clock registration and cut one more
dependency to plat-versatile.

This also makes this sched_clock implementation compulsory.

Cc: Will Deacon <will.deacon@arm.com>
Cc: Jonathan Austin <jonathan.austin@arm.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>