ARM: IXP4xx: 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: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Kevin Hilman <khilman@linaro.org>