ARM: ux500: use __iomem pointers for MMIO

In the earlier sweeping changes, the ux500 uncompress.h file was missed
because other problems were hiding this one.

Without this patch, building u8500_defconfig results in:

In file included from arch/arm/boot/compressed/misc.c:33:0:
arch/arm/mach-ux500/include/mach/uncompress.h: In function 'putc':
arch/arm/mach-ux500/include/mach/uncompress.h:32:2: warning: passing argument 1 of '__raw_readb' makes pointer from integer without a cast [enabled by default]
arch/arm/include/asm/io.h:95:89: note: expected 'const volatile void *' but argument is of type 'u32'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Linus Walleij <linus.walleij@linaro.org>