configs/chromebook_elm: bump to Linux v5.9

configs/chromebook_elm: bump to Linux v5.9

Update the chromebook elm configuration to use v5.9 instead of an
intermediate release candidate.

At the latter stages of the v5.9 release, a patch was added which
broke the Chromebook Elm HDMI. Therefore add a revert patch to get rid
of this problem. The fix (and the revert) are already on the mailing
list (https://lkml.org/lkml/2020/10/10/32) and should become available
with v5.10

Signed-off-by: Bilal Wasim <bilal.wasim@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>