BACKPORT: clocksource: Use GENMASK_ULL in definition of CLOCKSOURCE_MASK

Besides reusing existing code this removes the special case handling
for 64-bit masks, which causes clang to raise a shift count overflow
warning due to https://bugs.llvm.org//show_bug.cgi?id=10030

.

Suggested-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Cc: Grant Grundler <grundler@chromium.org>
Cc: Greg Hackmann <ghackmann@google.com>
Cc: Michael Davidson <md@google.com>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170418233037.70990-1-mka@chromium.org


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
(cherry picked from commit 0773cea37470f8e080c510fe720fc356cf35df3a)
[astrachan: minor re-diff; (cycle_t) vs (u64) cast]
Bug: 78886293
Change-Id: I97df9621cc65bba74a5086c2c35c2224b559a156
Signed-off-by: Alistair Strachan <astrachan@google.com>