clocksource: arm_global_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. While we're here increase the number of bits that
sched_clock can handle to 64 to make full use of the counter.

Cc: Stuart Menefy <stuart.menefy@st.com>
Cc: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Acked-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>