microblaze: Fix mapin_ram function

Fix how many pages are allocated in mapin_ram. It is lowmem_size
not setup CONFIG_LOWMEM_SIZE because it is the same for all systems.
Which means that wrong pages are allocated if memory size is smaller
than CONFIG_LOWMEM_SIZE. It has dramatic impact on bootup time.
On sp605 MMU full hw design is 7s.

Signed-off-by: Michal Simek <monstr@monstr.eu>
diff --git a/arch/microblaze/include/asm/page.h b/arch/microblaze/include/asm/page.h
index 665f293..352cc23 100644
--- a/arch/microblaze/include/asm/page.h
+++ b/arch/microblaze/include/asm/page.h
@@ -136,6 +136,7 @@
 
 extern unsigned long memory_start;
 extern unsigned long memory_size;
+extern unsigned long lowmem_size;
 
 extern int page_is_ram(unsigned long pfn);