imx5-ccm: clean up imx51_clocks_init

same as usual - spread out, reorder register for better readability

added a note about usb_phy_pred/podf since we reparent these to the OSC
clock in fixups (previously underneath where this code was cleaned up)
registering these two clocks makes no sense and adds two odd clocks off
PLL3 which could.. just go away silently.

That is unless a 27MHz clock is totally useful for the UTMI PHY vs. a
24MHz one (..?) and if PLL3 isn't going to be used. Might make an example
of this later for configurability of the clock tree (I wonder how much
memory a clock takes up in common clock subsystem?)
imx5-ccm: clean up the rest of the functions and tables
imx5-ccm: update comment