linux-purism-librem5: Add 166MHz to DDRC OPP table (MR 3754)

Backport the Purism patch from https://source.puri.sm/Librem5/linux/-/merge_requests/640



"With this patch, older revisions (2.0) should be able to switch between
166MHz and 800MHz as opposed to being stuck to 800MHz all the time, since
that was the only supported frequency in the OPP table."

Signed-off-by: Alistair Francis <alistair@alistair23.me>

[ci:skip-build] already built successfully in CI