clocksource: sirf: Switch to sched_clock_register() and use 64 bits

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 and use all 64 bits of this timer.

Cc: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>