fbdev: pxa3xx: use ktime_get_ts64 for time stamps

do_gettimeofday() is deprecated because it is not y2038 safe, so I'm
changing the calculation for the diagnostic output over to using
'timespec64'.

We really only print time deltas here, so changing it to monotonic
time makes this more robust, the correct accessor for this is
ktime_get_ts64().

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: Himanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>