ia64: efi: use timespec64 for persistent clock

We have a generic read_persistent_clock64 interface now, and can
change the ia64 implementation to provide that instead of
read_persistent_clock.

The main point of this is to avoid the use of struct timespec
in the global efi.h, which would cause build errors as soon
as we want to build a kernel without 'struct timespec' defined
on 32-bit architectures.

Aside from this, we get a little closer to removing the
__weak read_persistent_clock() definition, which relies on
converting all architectures to provide read_persistent_clock64
instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>