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>

include/asm-ppc/page.h

@@ -20,6 +20,7 @@
 /* This must match what is in arch/ppc/Makefile */
 #define PAGE_OFFSET	CONFIG_KERNEL_START
 #define KERNELBASE	PAGE_OFFSET
+#define is_kernel_addr(x)	((x) >= PAGE_OFFSET)
 
 #ifndef __ASSEMBLY__