microblaze: Fix sparse warning - cpuinfo.h

Warning log:
CHECK   arch/microblaze/kernel/cpu/cpuinfo-static.c
arch/microblaze/include/asm/cpuinfo.h:101:21: warning: incorrect type in argument 1 (different signedness)
arch/microblaze/include/asm/cpuinfo.h:101:21:    expected unsigned int const [usertype] *p
arch/microblaze/include/asm/cpuinfo.h:101:21:    got int *[assigned] val
...

Signed-off-by: Michal Simek <monstr@monstr.eu>