powerpc: Constify struct kobj_type

powerpc: Constify struct kobj_type

'struct kobj_type' is not modified. It is only used in
kobject_init_and_add()/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
   7145	    606	      0	   7751	   1e47	arch/powerpc/kernel/cacheinfo.o
   3663	    384	     16	   4063	    fdf	arch/powerpc/kernel/secvar-sysfs.o

After:
======
   text    data     bss     dec     hex filename
   7193	    558	      0	   7751	   1e47	arch/powerpc/kernel/cacheinfo.o
   3663	    384	     16	   4063	    fdf	arch/powerpc/kernel/secvar-sysfs.o

Signed-off-by: Huang Xiaojia <huangxiaojia2@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240826150957.3500237-1-huangxiaojia2@huawei.com