bcma: add bcma_core_pci_extend_L1timer

This code is based on code from pcie_extendL1timer() in brcmsmac. This
patch is part of the move of pci specific code from brcmsmac to bcma.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>