microblaze: Use proper casting for inb/inw/inl in io.h

We are going to move to asm-generic/io.h but
let's fix compilation warnings first for 3.10.

Warning message:
arch/microblaze/include/asm/io.h:126:26: warning: cast to
 pointer from integer of different size [-Wint-to-pointer-cast]
 #define inb(port)  readb((u8 *)((port)))
...

Signed-off-by: Michal Simek <michal.simek@xilinx.com>