mtd: add arch dependency for MTD_BCM47XXSFLASH symbol

We dropped strict MIPS dependency for bcm47xxsflash driver in:
commit 5651d6aa ("mtd: bcm47xxsflash: use ioremap_cache() instead of
KSEG0ADDR()") but using ioremap_cache still limits building it to few
selected architectures only.

A recent commit 57d8f7dd ("bcma: allow enabling serial flash support
on non-MIPS SoCs") automatically dropped MIPS dependency for
MTD_BCM47XXSFLASH which broke building e.g. on powerpc and cris.

The bcma change is alright as it doesn't break building bcma code in any
way. MTD_BCM47XXSFLASH on the other hand should be limited to archs
which need it and can build it (by providing ioremap_cache).

Fixes: 57d8f7dd ("bcma: allow enabling serial flash support on non-MIPS SoCs")
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>