clocksource: vf_pit_timer: Switch to sched_clock_register()

The 32 bit sched_clock interface now supports 64 bits. Upgrade to
the 64 bit function to allow us to remove the 32 bit registration
interface.

Cc: Jingchang Lu <b35083@freescale.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>