[2nd try] Switch the i.MX27's PLL in a safe manner

[Juergen Beisert]

(hopefully this time with *all* files)

This patch is required to switch the PLL of the i.MX27 at runtime without data
loss and random system crashes (but it is only required in this way if the CPU
core does not run at 1.45 V immediatly after reset).

Currently related only to the phyCORE-i.MX27 platform.