init/main.c: add sync point between each level

With PROBE_PREFER_ASYNCHRONOUS, drivers have ability to do asynchronous
probing, however this might break promise of ordering. This patch is to
add additional sync point between each init level.

Test: Boot
Bug: 63716230
Bug: 77146523
Change-Id: Ie6282bb64b8bc8fe239748de78e2c64a28d51bd2
Signed-off-by: Wei Wang <wvw@google.com>