bcma: prepare Kconfig symbol for PCI driver

Driver for PCIe core requires PCI to be enabled, however we shouldn't
require it for the whole bus. Someone may be not interested in extra
PCI devices and what's more there are SoCs without any PCI at all (like
BCM5356C0, BCM5357*, BCM47186B0). For more details see Kconfig "help".
Please note this patch doesn't allow disabling PCI drivers yet, as it
requires more work on calls to bcma_core_pci_* functions.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>