The Exynos ChipID driver, like most of the Exynos drivers, uses one
compatible for entire family of compatible devices using one devicetree
"compatible".  The compatibility is here described by programming
interface (register layout), not by actual values, so the product ID
register on one family of devices has different values for different
SoCs.

Describe which SoC goes with which compatible for documentation
purposes, if the DTS is not available in mainline.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20211031205212.59505-1-krzysztof.kozlowski@canonical.com