riscv: remove volatile from set_gd prototype

It's slightly ironic that riscv at very first had the gd pointer
volatile qualified [6020faf62c ("riscv: nx25: include: Add header
files to support RISC-V")], removed that back in 2018
[40717eb849 ("riscv: checkpatch: Fix use of volatile")], and then in
2020 this helper was added [6b9966e1aa ("riscv: define function
set_gd()")] which needlessly had volatile in the prototype.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>

arch/riscv/include/asm/global_data.h

@@ -68,7 +68,7 @@ static inline gd_t *get_gd(void)
 
 #endif
 
-static inline void set_gd(volatile gd_t *gd_ptr)
+static inline void set_gd(gd_t *gd_ptr)
 {
 #ifdef CONFIG_64BIT
 	asm volatile("ld gp, %0\n" : : "m"(gd_ptr));