platform/x86: thinkpad_acpi: Add support for dual fan control

This adds dual fan control for the following models:
P50, P51, P52, P70, P71, P72, P1 gen1, P2 gen2, X1E gen1 and X1E gen2.

Both fans are controlled together as if they were a single fan.

Tested on an X1 Extreme Gen1, an X1 Extreme Gen2, and a P50.

The patch is defensive, it adds only specific supported machines, and falls
back to the old behavior if both fans cannot be controlled.

Background:
I tested the BIOS default behavior on my X1E gen2 and both fans are always
changed together. So rather than adding controls for each fan, this controls
both fans together as the BIOS would do.

This was inspired by a discussion on dual fan support for the thinkfan tool
(see link below). All BIOS IDs are taken from there. The X1E gen2 ID is
verified on my machine.

Thanks to GitHub users voidworker and civic9 for the earlier patches and
BIOS IDs, and to users peter-stoll and sassman for testing the patch on
their machines.

BugLink: https://github.com/vmatare/thinkfan/issues/58


Signed-off-by: Lars Hofhansl <larsh@apache.org>
[andy: massaged commit message to capitalize ID and convert to BugLink]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>