On coachz it could be observed that SPI_CLK voltage level was only
1.4V during active transfers because the drive strength was too
weak. The line hadn't finished slewing up by the time we started
driving it down again. Using a drive strength of 8 lets us achieve the
correct voltage level of 1.8V.

Though the worst problems were observed on coachz hardware, let's do
this across the board for trogdor devices. Scoping other boards shows
that this makes the clk line look nicer on them too and doesn't
introduce any problems.

Only the clk line is adjusted, not any data lines. Because SPI isn't a
DDR protocol we only sample the data lines on either rising or falling
edges, not both. That means the clk line needs to toggle twice as fast
as data lines so having the higher drive strength is more important
there.
Signed-off-by: Wenchao Han <hanwenchao@huaqin.corp-partner.google.com>
[dianders: Adjust author real name; adjust commit message]
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20210510075253.1.Ib4c296d6ff9819f26bcaf91e8a08729cc203fed0@changeidSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>