arm64: dts: rockchip: Specify override mode for kevin panel

This patch adds an override mode for kevin devices. The mode increases
both back porches to allow a pixel clock of 26666kHz as opposed to the
'typical' value of 252750kHz. This is needed to avoid interference with
the touch digitizer on these laptops.

Cc: Doug Anderson <dianders@chromium.org>
Cc: Eric Anholt <eric@anholt.net>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Jeffy Chen <jeffy.chen@rock-chips.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Stéphane Marchesin <marcheu@chromium.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: devicetree@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-rockchip@lists.infradead.org
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>