bcma: allow enabling serial flash support on non-MIPS SoCs

So far we had only MIPS devices with serial flash connected to the SoC's
ChipCommon. ARM devices got a separated SPI controller and weere using
standard SPI drivers.
This has changed with the wireless SoC BCM47189B0. It's ARM based but
has serial flash attached just like older devices. This allows using
existing driver with these devices.

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