dt: Drop I2C for Pi4 HDMI interfaces to 97.5kHz.

It was set to 390kHz, which is outside of the required spec for
reading HDMI (max 100kHz). The i2c-brcmstb driver only supports
a number of fixed bus speeds, of which 97.5kHz is the closest to
100kHz without exceeding it.

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