powerpc/mpc85xx: Reset the PCIe bus on attach

It seems if a Radeon card is already initialized by u-boot, it won't be
reinitialized by the kernel, and the DRM module will fail to attach. This
steals the reset code from mips/octopci.c to blindly reset the bus on attach.
This was tested on a AmigaOne X5000/20, such that it can be booted from the
local video console, and get a video console in FreeBSD.