allwinner: Add a new clock aw_clk_m

We used the aw_clk_nm clock for clock with only one divider factor
and used a fake multiplier factor. This cannot work properly as we
end up writing the "fake" factor to the register (and so always set
the LSB to 1).
Create a new clock for those.
The reason for not using the clk_div clock is because those clocks are
a bit special. Since they are (almost) all related to video we also need
to set the parent clock (the main PLL) to a frequency that they can support.
As the main PLL have some minimal frequency that they can support we need to
be able to set the main PLL to a multiple of the desired frequency.
Let say you want to have a 71Mhz pixel clock (typical for a 1280x800 display)
and the main PLL cannot go under 192Mhz, you need to set it to 3 times the
desired frequency and set the divider to 3 on the hdmi clock.
So this also introduce the CLK_SET_ROUND_MULTIPLE flag that allow for this kind
of scenario.