dt-bindings: msm: dsi-controller-main: Rename qcom,dsi-ctrl-6g-qcm2290 to qcom,qcm2290-dsi-ctrl

We will add in a number of compat strings to dsi-controller-main.yaml in
the format "qcom,socname-dsi-ctrl" convert the currently unused
qcom,dsi-ctrl-6g-qcm2290 to qcom,qcm2290-dsi-ctrl.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>