bcma: move flash detection code to ChipCommon core driver

Flashes are attached and accessible through ChipCommon core, they aren't
connected to MIPS one. Moreover some new ARM devices (e.g. BCM47189) may
have serial flash accessibility using ChipCommon registers as well. To
support them we can't keep this code in MIPS core driver.

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