servod: mimic flex: change to allow either pca9500 or pca9537.

For electrical reasons (see tracker) some DUT devices cannot tolerate
the current source included in the pca9500.  To alleviate that issue
all future mimic flex cables will employ the pca9534.

This CL allows both old & new mimic cables to work by creating a new
virtual driver pca95xx and dynamically determining which flex is
installed.

BUG=chromium:233244
TEST=manual
run servod w/ old (PCA9500) and new (PCA9534) mimic flexes and
successfully read the f/w from Spring which MUST set spi2_buf_on_flex.

sudo servod -b spring
dut-control spi2_buf_en:on spi2_buf_on_flex_en:on spi2_vref:pp1800
sudo flashrom -p ft2232_spi:type=servo-v2 -r /tmp/fw.bin
dut-control spi2_buf_en:off spi2_buf_on_flex_en:off spi2_vref:off

Change-Id: I85d28220bfa42b585c22d0d4b9d423b9b07ebbed
Reviewed-on: https://gerrit.chromium.org/gerrit/48531
Tested-by: Todd Broch <tbroch@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
Commit-Queue: Todd Broch <tbroch@chromium.org>
3 files changed