emul: icm42688: Implement backend sensor emul API

Implement the backend emul API for the ICM42688 motion sensor so it can
be automatically tested by the generic sensor test (see #60394).
Supports all channels (temp, accel XYZ, and gyro XYZ) at each of the
programmable full-scale accel and gyro ranges.

Also fixes an arithmetic bug in the driver that was causing a minor
error in the returned readings.

Signed-off-by: Tristan Honscheid <honscheid@google.com>