riscv: checkpatch: Fix use of volatile

It is reported by checkpatch.pl
WARNING: Use of volatile is usually wrong: see
Documentation/process/volatile-considered-harmful.rst

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>

arch/riscv/include/asm/global_data.h

@@ -17,6 +17,6 @@ struct arch_global_data {
 
 #include <asm-generic/global_data.h>
 
-#define DECLARE_GLOBAL_DATA_PTR register volatile gd_t *gd asm ("gp")
+#define DECLARE_GLOBAL_DATA_PTR register gd_t *gd asm ("gp")
 
 #endif /* __ASM_GBL_DATA_H */