arm: dt: bcm2712: Reduce DDC frequency to 97.5kHz from 200kHz.

The I2C spec says the DDC link should run at 100kHz or less, however
Pi5/BCM2712 had been configured for 200kHz.
Reduce it to comply with the spec, and match Pi4.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>