cr50: make EC RST synchronous
cr50: make EC RST synchronous

When the RBOX register controlling the EC reset output is written,
RBOX does not act immediately, as it is controlled by its own clock.
This results in the program continuing executing before RBOX output
value change.

This patch makes sure that execution does not continue until RBOX
reacted to the request to change the EC RST output value.

BRANCH=cr50. cr50-mp
BUG=b:75976718
TEST=verified with logic analyzer that processing does not continue
     until EC RST output value changes.

Change-Id: I72814d3c9ea11721e1b361e7f6d300658306562d
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1214101
Reviewed-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec