mtd: bcm47xxsflash: use ioremap_cache() instead of KSEG0ADDR()

mtd: bcm47xxsflash: use ioremap_cache() instead of KSEG0ADDR()

Using KSEG0ADDR makes code highly MIPS dependent and not portable.
Thanks to the fix a68f376 ("MIPS: io.h: Define `ioremap_cache'") we can
use ioremap_cache which is generic and supported on MIPS as well now.

KSEG0ADDR was translating 0x1c000000 into 0x9c000000. With ioremap_cache
we use MIPS's __ioremap (and then remap_area_pages). This results in
different address (e.g. 0xc0080000) but it still should be cached as
expected and it was successfully tested with BCM47186B0.

Other than that drivers/bcma/driver_chipcommon_sflash.c nicely setups a
struct resource for access window, but we wren't using it. Use it now
and drop duplicated info.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>