powerpc/32: Define an is_kernel_addr() to fix ARCH=ppc compilation

My commit 6bfd93c3

 broke the ARCH=ppc
compilation by using the is_kernel_addr() macro in asm/uaccess.h.
This fixes it by defining a suitable is_kernel_addr() for ARCH=ppc.

Signed-off-by: Paul Mackerras <paulus@samba.org>