nds32: Fix vDSO clock_getres()

clock_getres in the vDSO library has to preserve the same behaviour
of posix_get_hrtimer_res().

In particular, posix_get_hrtimer_res() does:
    sec = 0;
    ns = hrtimer_resolution;
and hrtimer_resolution depends on the enablement of the high
resolution timers that can happen either at compile or at run time.

Fix the nds32 vdso implementation of clock_getres keeping a copy of
hrtimer_resolution in vdso data and using that directly.

Cc: Greentime Hu <green.hu@gmail.com>
Cc: Vincent Chen <deanbo422@gmail.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>

arch/nds32/include/asm/vdso_datapage.h

@@ -20,6 +20,7 @@ struct vdso_data {
 	u32 xtime_clock_sec;	/* CLOCK_REALTIME - seconds */
 	u32 cs_mult;		/* clocksource multiplier */
 	u32 cs_shift;		/* Cycle to nanosecond divisor (power of two) */
+	u32 hrtimer_res;	/* hrtimer resolution */
 
 	u64 cs_cycle_last;	/* last cycle value */
 	u64 cs_mask;		/* clocksource mask */