dt-bindings: display: vop2: Add optional PLL clock property for rk3576

As with the RK3588 SoC, RK3576 also allows the use of HDMI PHY PLL as an
alternative and more accurate pixel clock source for VOP2.

Document the optional PLL clock property.

Moreover, given that this is part of a series intended to address some
recent display problems, provide the appropriate tags to facilitate
backporting.

Fixes: c3b7c5a4 ("dt-bindings: display: vop2: Add rk3576 support")
Cc: stable@vger.kernel.org
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: "Rob Herring (Arm)" <robh@kernel.org>
Tested-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250612-rk3576-hdmitx-fix-v1-1-4b11007d8675@collabora.com