dt-bindings: msm: dsi: Fix phy-name binding

The driver doesn't care about the name of the phy in DTS. msm8916 has since
2016 named the phy "dsi-phy" with later SoCs taking the name "dsi".

There's no reason to have a constraint on the name here, simply to have a
name. Remove the redundant const in favour of maxItems: 1.

Fixes: 4dbe55c9 ("dt-bindings: msm: dsi: add yaml schemas for DSI bindings")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>