wcnss: Add API to get monotonic boot time

Add WCNSS platform driver API to expose monotonic boot time so that
WLAN host driver can get the timestamp.

Change-Id: Id1db5bc7efac036cf76d2090ab5c07d2aece5e1a
CRs-Fixed: 706813
Signed-off-by: Hardik Kantilal Patel <hkpatel@codeaurora.org>