pps-gpio: Timestamp alignment with boottime

GPS driver which makes use of PPS timestamp expects time
with respect to boot time. Existing pps_get_ts function
doesn't return time from the boot so replaced this function
with get_monotonic_boottime function which returns elapsed time
from the boot.

Change-Id: I8f656a1c4ef43f6314d4ab295a177125bcc8195e
CRs-Fixed: 1012438
Signed-off-by: Naresh Munagala <nareshm@codeaurora.org>