riscv: remove volatile from set_gd prototype

riscv: remove volatile from set_gd prototype

It's slightly ironic that riscv at very first had the gd pointer
volatile qualified [6020faf62c3 ("riscv: nx25: include: Add header
files to support RISC-V")], removed that back in 2018
[40717eb849c ("riscv: checkpatch: Fix use of volatile")], and then in
2020 this helper was added [6b9966e1aa3 ("riscv: define function
set_gd()")] which needlessly had volatile in the prototype.

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