[ARM] MXC: Use a single function for decoding a PLL

We had 3 versions of this function in clock support for MX1/2/3
Use a single one instead. I picked the one from the MX3 as it seems
to calculate more accurate as the other ones. Also, on MX27 and MX31 mfn
can be negative, this hasn't been handled correctly on MX27 since now.

This patch has been tested on MX27 and MX31 and produces the same clock
frequencies for me.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>