bcma: gpio: use predictable base for all BCM47XX buses

Some BCM47XX SoC routers have LEDs connected to extra PCIe bcma buses.
Handling them in arch code requires predictable GPIO numbers.

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