x86: Refactor PCI to permit alternate init

We want access PCI earlier in the init sequence, so refactor the code so
that it does not require use of a BSS variable to work. This will allow us
to use early malloc() to store information about a PCI hose.

Common PCI code moves to arch/x86/cpu/pci.c and a new
board_pci_setup_hose() function is provided by boards to set up the (single)
hose used by that board.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>