ARM: dts: am437x-gp: switch i2c0 to 100KHz

On the GP EVM, the ambient light sensor is limited to 100KHz on the
I2C bus.

So use 100kHz for I2C on the GP EVM due to this limitation on the
ambient light sensor.

Reported-by: Aparna Balasubramanian <aparnab@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>