clk: max77686: Add DT binding details for PMIC MAX77620

Maxim has used the same clock IP on multiple PMICs like MAX77686,
MAX77802, MAX77620. Only differences are the number of clocks
from these PMICs like MAX77686 has 3 clocks output, MAX776802 have
two clock output and MAX77620 has one clock output.

Add clock binding details and DT example for the MAX77620.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
CC: Krzysztof Kozlowski <k.kozlowski@samsung.com>
CC: Javier Martinez Canillas <javier@dowhile0.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>