drivers: rtc: Add delay to wait for new config to take effect

Add delay to wait for new configuration to take effect before
returning to API caller. It takes time for any change to rtc
configuration to take effect since rtc runs under a much slower
clock.

Of course, we do not have an equation for best wait time.
The value is based on the test results on arduino 101, minfield
and quark se c1000 devboard(some margin added).

Jira: ZEP-991

Change-Id: I0dcee3c5809963dcd418186b85e3473427a4d526
Signed-off-by: Baohong Liu <baohong.liu@intel.com>