x86: coreboot: Move non-board specific files to coreboot arch directory

coreboot.c and coreboot_pci.c don't contain board specific but only
coreboot specific code. Hence move it to the coreboot directory in
arch/x86/cpu (which should probably be moved out of cpu/ in another
commit)

Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>