armv8: enable compilation with CONFIG_SYS_NONCACHED_MEMORY

The implementation of noncached_init() uses define MMU_SECTION_SIZE.
Define this on ARM64.

Move the prototype of noncached_{init,alloc}() to a location that
doesn't depend on !defined(CONFIG_ARM64).

Note that noncached_init() calls mmu_set_region_dcache_behaviour() which
relies on something having set up translation tables with 2MB block size.
The core ARMv8 MMU setup code does not do this by default, but currently
relies on SoC specific MMU setup code. Be aware of this before enabling
this feature on your platform!

Signed-off-by: Stephen Warren <swarren@nvidia.com>

arch/arm/include/asm/system.h

@@ -17,6 +17,7 @@
 #define PGTABLE_SIZE	(0x10000)
 /* 2MB granularity */
 #define MMU_SECTION_SHIFT	21
+#define MMU_SECTION_SIZE	(1 << MMU_SECTION_SHIFT)
 
 #ifndef __ASSEMBLY__
 
@@ -278,11 +279,6 @@ enum {
  */
 void mmu_page_table_flush(unsigned long start, unsigned long stop);
 
-#ifdef CONFIG_SYS_NONCACHED_MEMORY
-void noncached_init(void);
-phys_addr_t noncached_alloc(size_t size, size_t align);
-#endif /* CONFIG_SYS_NONCACHED_MEMORY */
-
 #endif /* __ASSEMBLY__ */
 
 #define arch_align_stack(x) (x)
@@ -302,6 +298,11 @@ phys_addr_t noncached_alloc(size_t size, size_t align);
 void mmu_set_region_dcache_behaviour(phys_addr_t start, size_t size,
 				     enum dcache_option option);
 
+#ifdef CONFIG_SYS_NONCACHED_MEMORY
+void noncached_init(void);
+phys_addr_t noncached_alloc(size_t size, size_t align);
+#endif /* CONFIG_SYS_NONCACHED_MEMORY */
+
 #endif /* __ASSEMBLY__ */
 
 #endif