OMAP: omap_device: use read_persistent_clock() instead of getnstimeofday()

During suspend and resume, when omap_device deactivation and
activation is happening, the timekeeping subsystem has likely already
been suspended.  Thus getnstimeofday() will fail and trigger a WARN().

Use read_persistent_clock() instead of getnstimeofday() to avoid this.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>