arm:d2plug: Add support for AK8128 clock generator

D2plug uses AK8128 HDMI clock generator which can generate five
different frequencies:
1. 27 MHz
2. 74.25 MHz
3. 74.25/1.001 MHz
4. 148.5 MHz
5. 148.5/1.001 MHz

AK8128 is controlled by simple GPIO logic which is implemented as a
separate clock source ("hdmi_clk") for LCD0.

Signed-off-by: Ajay Bhargav <bhargav.ajay@gmail.com>