dt-bindings: clk: mstar msc313 mpll binding description

Add a binding description for the MStar/SigmaStar MPLL clock block.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/r/20210211052206.2955988-3-daniel@0x0f.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>