s390/time: Add read_persistent_wall_and_boot_offset()

s390/time: Add read_persistent_wall_and_boot_offset()

read_persistent_wall_and_boot_offset() will replace read_boot_clock64()
because on some architectures it is more convenient to read both sources
as one may depend on the other. For s390, implementation is the same
as read_boot_clock64() but also calling and returning value of
read_persistent_clock64()

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: steven.sistare@oracle.com
Cc: daniel.m.jordan@oracle.com
Cc: linux@armlinux.org.uk
Cc: heiko.carstens@de.ibm.com
Cc: john.stultz@linaro.org
Cc: sboyd@codeaurora.org
Cc: hpa@zytor.com
Cc: douly.fnst@cn.fujitsu.com
Cc: peterz@infradead.org
Cc: prarit@redhat.com
Cc: feng.tang@intel.com
Cc: pmladek@suse.com
Cc: gnomes@lxorguk.ukuu.org.uk
Cc: linux-s390@vger.kernel.org
Cc: boris.ostrovsky@oracle.com
Cc: jgross@suse.com
Cc: pbonzini@redhat.com
Link: https://lkml.kernel.org/r/20180719205545.16512-15-pasha.tatashin@oracle.com