powerpc: powernv: Constify struct kobj_type

powerpc: powernv: 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
   3775	    256	      8	   4039	    fc7	arch/powerpc/platforms/powernv/opal-dump.o
   2679	    260	      8	   2947	    b83	arch/powerpc/platforms/powernv/opal-elog.o

After:
======
   text	   data	    bss	    dec	    hex	filename
   3823	    208	      8	   4039	    fc7	arch/powerpc/platforms/powernv/opal-dump.o
   2727	    212	      8	   2947	    b83	arch/powerpc/platforms/powernv/opal-elog.o

Signed-off-by: Huang Xiaojia <huangxiaojia2@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240826150957.3500237-2-huangxiaojia2@huawei.com