ppc4xx: Make pll_write global

This patch makes pll_write on PPC405EP boards
global and callable from C code.

pll_write can be used to dynamically modify the PLB:PCI divider
as it is required for 33/66 MHz pci adapters based on the 405EP.

board_early_init_f() is a good place to do that (check M66EN signal
and call pll_write() when it is required).

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Signed-off-by: Stefan Roese <sr@denx.de>