xtensa: drop dead PCI support code

xtensa-specific PCI initialization code has significantly bitrotted over
time because there's no platform that use it. Get rid of remaining
non-functioning initialization and remove platform_pcibios_* interface.
A new platform that would use PCI on xtensa will configure PCI
controller using device tree.

Drop variables pci_ctrl_head, pci_bus_count and functions pcibios_init,
pci_controller_apertures, platform_pcibios_init and
platform_pcibios_fixup.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>