bcma: add method to power up and down the PCIe core by wifi driver

The wifi driver should tell the PCIe core that it is now in operation
so that some workarounds can be applied and the power state is changed.
This should replace the call to bcma_core_pci_extend_L1timer by the
brcmsmac driver.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>