wcnss: Add API to get monotonic boot time

Add CNSS platform driver API to expose monotonic boot time so that
WLAN host driver can get the timestamp.
Change-Id: I722fa42600bf910254ac01a7305bbbaa35dfa83e
CRs-Fixed: 706813
Signed-off-by: Hardik Kantilal Patel <hkpatel@codeaurora.org>