powerpc: xmon: use ktime_get_coarse_boottime64

get_monotonic_boottime() is deprecated, and may not be safe to call in
every context, as it has to read a hardware clocksource.

This changes xmon to print the time using ktime_get_coarse_boottime64()
instead, which avoids the old timespec type and the HW access.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>