dt-bindings: clk: mstar msc313 mpll binding header

Simple header to document the relationship between the MPLL outputs
and which divider they come from.

Output 0 is missing because it should not be consumed.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210211052206.2955988-2-daniel@0x0f.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>