seco-mx8mm: Allow to specify the VIDIO_PLL1_RATE as define

This is a bit hacky but currently I don't know a better solution.

Though the doc say assigned-clock-parents may not be used to set clock rates
if there are multiple consumers this is used here widely.
( see Documentation/devicetree/bindings/clock/clock-bindings.txt line 164)
To allow to change this in devicetree using this file, this macro is
introduced, using the default value from before as default.

BCS 746-001034