powerpc: pseries: Constify struct kobj_type

powerpc: pseries: Constify struct kobj_type

'struct kobj_type' is not modified. It is only used in kobject_init()
which takes a 'const struct kobj_type *ktype' parameter.

Constifying this structure moves some data to a read-only section,
so increase over all security.

On a x86_64, compiled with ppc64 defconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   1885	    368	     16	   2269	    8dd	arch/powerpc/platforms/pseries/vas-sysfs.o

After:
======
   text	   data	    bss	    dec	    hex	filename
   1981	    272	     16	   2269	    8dd	arch/powerpc/platforms/pseries/vas-sysfs.o

Signed-off-by: Huang Xiaojia <huangxiaojia2@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240826150957.3500237-3-huangxiaojia2@huawei.com