powerpc/8xx: Define a MODULE area below kernel text

On the 8xx, TASK_SIZE is 0x80000000. The space between TASK_SIZE
and PAGE_OFFSET is not used.

In order to benefit from the powerpc specific module_alloc()
function which allocate modules with 32 Mbytes from
end of kernel text, define MODULES_VADDR and MODULES_END.

Set a 256Mb area just below PAGE_OFFSET, like book3s/32.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a225606d5b3a8bc53fe612ad52c855c60b0a0a58.1617283827.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/nohash/32/mmu-8xx.h

@@ -172,6 +172,9 @@
 
 #define mmu_linear_psize	MMU_PAGE_8M
 
+#define MODULES_VADDR	(PAGE_OFFSET - SZ_256M)
+#define MODULES_END	PAGE_OFFSET
+
 #ifndef __ASSEMBLY__
 
 #include <linux/mmdebug.h>