mhi: core: add asynchronous time request support in sysfs

Add support to request time from device using the asynchronous
method in ticks and microseconds using sysfs entries.

Change-Id: Idb661ad6db9f1cc4cd594c96755986ece4961b03
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>