riscv: fix "Cannot allocate SWIOTLB buffer"

* By default, ARCH_LOW_ADDRESS_LIMIT is 0xffff_ffffUL. When we put the
  kernel over 4GB physical memory, swiotlb can not get free memory below
  ARCH_LOW_ADDRESS_LIMIT. Modifying ARCH_LOW_ADDRESS_LIMIT to
  0xf_ffff_ffffUL can fix this issue.

arch/riscv/include/asm/processor.h

@@ -92,6 +92,8 @@ extern int riscv_of_processor_hart(struct device_node *node);
 
 extern void riscv_fill_hwcap(void);
 
+#define ARCH_LOW_ADDRESS_LIMIT  (0xfffffffffUL)
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_RISCV_PROCESSOR_H */