pps-gpio: Add new property to use system time timestamp for PPS GPIO

If the device tree property "use-system-time-ts" is defined
then the timestamp used for PPS events will be the system
time, otherwise it will be the monotonic boot time.

Change-Id: I92fab1d8861e1d652fcb68066b417f44c1f8e29b
Signed-off-by: Gustavo Solaira <gustavos@codeaurora.org>