MIPS: BCM63XX: add and use a clock for PCIe

Add a PCIe clock and use that instead of directly touching the clock
control register. While at it, fail if there is no such clock.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4452
Signed-off-by: John Crispin <blogic@openwrt.org>