arm64: mm: make CONFIG_ZONE_DMA32 configurable

This change makes CONFIG_ZONE_DMA32 defuly y and allows users
to overwrite it only when CONFIG_EXPERT=y.

For the SoCs that do not need CONFIG_ZONE_DMA32, this is the
first step to manage all available memory by a single
zone(normal zone) to reduce the overhead of multiple zones.

The change also fixes a build error when CONFIG_NUMA=y and
CONFIG_ZONE_DMA32=n.

arch/arm64/mm/init.c:195:17: error: use of undeclared identifier 'ZONE_DMA32'
                max_zone_pfns[ZONE_DMA32] = PFN_DOWN(max_zone_dma_phys());

Change since v1:
1. only expose CONFIG_ZONE_DMA32 when CONFIG_EXPERT=y
2. remove redundant IS_ENABLED(CONFIG_ZONE_DMA32)

Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -260,7 +260,8 @@ config GENERIC_CALIBRATE_DELAY
 	def_bool y
 
 config ZONE_DMA32
-	def_bool y
+	bool "Support DMA32 zone" if EXPERT
+	default y
 
 config HAVE_GENERIC_GUP
 	def_bool y

arch/arm64/mm/init.c

@@ -191,8 +191,9 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
 {
 	unsigned long max_zone_pfns[MAX_NR_ZONES]  = {0};
 
-	if (IS_ENABLED(CONFIG_ZONE_DMA32))
-		max_zone_pfns[ZONE_DMA32] = PFN_DOWN(max_zone_dma_phys());
+#ifdef CONFIG_ZONE_DMA32
+	max_zone_pfns[ZONE_DMA32] = PFN_DOWN(max_zone_dma_phys());
+#endif
 	max_zone_pfns[ZONE_NORMAL] = max;
 
 	free_area_init_nodes(max_zone_pfns);