google/gru: Determine Board ID based on the input voltage of ADC1

The Board ID on the Gru family of boards is determined by reading the
voltage from a resistor divider, each hardware revision is supposed to
have a unique resistor ratio, which allows to distinctly tell between
different Board ID.

While the long time approach to mapping resistor ratios (and voltages)
into Board ID remains under discussion, we know for sure the values
for Proto 1 and Proto 2. Let's just use them for now.

Since Board ID can be queried multiple times during boot, ideally it
should be read once and placed in the coreboot table to be available
to all coreboot stages. For now we just cache it so that at least
during the same stage the ADC has to run only once.

BRANCH=None
BUG=chrome-os-partner:51537
TEST=verified that the voltage reading on Proto 1 is as expected, and
     Board ID 0 is reported.

Change-Id: I94bc7fc235dae4155feb6ca35b5ef0ab20c3ec9c
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: bb4064d0af8174b6ae247cdad9378b7f4e5f22ba
Original-Change-Id: I105ea97f8862b5707b582904c6f2e3e9406a0f07
Original-Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/340428
Original-Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: https://review.coreboot.org/14722
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

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