arm64: dts: exynos: Increase bus frequency for MHL chip

sii8620 supports 1 MHz clock, it allows faster transmissions and according
to extensive tests allows to mitigate some obscure bugs in I2C client
logic of the chip.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>