mhi: core: add support to get external modem time

For accurate synchronizations between external
modem and host processor, mhi host will capture
modem time relative to host time. Client may use
time measurements for adjusting any drift between
host and modem.

CRs-Fixed: 2246653
Change-Id: I35fceea13e0df3929d4b0efd6b77a8de4211099c
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>