microblaze: mm: Fix lowmem max memory size limits

Use CONFIG_LOWMEM_SIZE if system has larger ram size.
For system with larger ram size, enable HIGMEM support.

Also setup limitation for memblock and use memblock
allocation in lowmem region.

Signed-off-by: Michal Simek <monstr@monstr.eu>
diff --git a/arch/microblaze/include/asm/pgtable.h b/arch/microblaze/include/asm/pgtable.h
index b2af423..d8f2c3c 100644
--- a/arch/microblaze/include/asm/pgtable.h
+++ b/arch/microblaze/include/asm/pgtable.h
@@ -94,8 +94,7 @@
 /* Start and end of the vmalloc area. */
 /* Make sure to map the vmalloc area above the pinned kernel memory area
    of 32Mb.  */
-#define VMALLOC_START	(CONFIG_KERNEL_START + \
-				max(32 * 1024 * 1024UL, memory_size))
+#define VMALLOC_START	(CONFIG_KERNEL_START + CONFIG_LOWMEM_SIZE)
 #define VMALLOC_END	ioremap_bot
 
 #endif /* __ASSEMBLY__ */