arm64: dts: imx8mq: add hdmi phy 27m clock

The i.MX8MQ PLL support hdmi phy 27m as pll reference clock,
so add a fixed clock for it.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>