hrtimer: hook compat_sys_nanosleep up to high res timer code

Now we have high res timers on ppc64 I thought Id test them. It turns
out compat_sys_nanosleep hasnt been converted to the hrtimer code and so
is limited to HZ resolution.

The follow patch converts compat_sys_nanosleep to use high res timers.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>