arm64: dts: librem5-devkit: add lsm9ds1 mount matrix

The IMU chip on the librem5-devkit is not mounted at the "natural" place
that would match normal phone orientation (see the documentation for the
details about what that is).

Since the lsm9ds1 driver supports providing a mount matrix, we can describe
the orientation on the board in the dts:

Create a right-handed coordinate system (x * -1; see the datasheet for the
axis) and rotate 180 degrees around the y axis because the device sits on
the back side from the display.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>