IXP4xx: use __iomem for MMIO

The ixp4xx queue manager uses "const struct qmgr_regs __iomem *" as the
type for a pointer that is passed to __raw_writel, which is not
allowed because of the const-ness.

Dropping the 'const' keyword fixes the problem. While we're here,
let's also drop the useless type cast.

Without this patch, building ixp4xx_defconfig results in:

In file included from arch/arm/mach-ixp4xx/ixp4xx_qmgr.c:15:0:
arch/arm/mach-ixp4xx/include/mach/qmgr.h: In function 'qmgr_put_entry':
arch/arm/mach-ixp4xx/include/mach/qmgr.h:96:2: warning: passing argument 2 of '__raw_writel' discards 'const' qualifier from pointer target type [enabled by default]
arch/arm/include/asm/io.h:88:91: note: expected 'volatile void *' but argument is of type 'const u32 *'
In file included from drivers/net/ethernet/xscale/ixp4xx_eth.c:41:0:
arch/arm/mach-ixp4xx/include/mach/qmgr.h: In function 'qmgr_put_entry':
arch/arm/mach-ixp4xx/include/mach/qmgr.h:96:2: warning: passing argument 2 of '__raw_writel' discards 'const' qualifier from pointer target type [enabled by default]
arch/arm/include/asm/io.h:88:91: note: expected 'volatile void *' but argument is of type 'const u32 *'
arch/arm/mach-ixp4xx/ixp4xx_qmgr.c: In function 'qmgr_set_irq':
arch/arm/mach-ixp4xx/ixp4xx_qmgr.c:41:9: warning: passing argument 2 of '__raw_writel' discards 'const' qualifier from pointer target type [enabled by default]
arch/arm/include/asm/io.h:88:91: note: expected 'volatile void *' but argument is of type 'const u32 *'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>