RISC-V: prefetch: const offset needs to have 5 bits zero, not 4

Spotted this by chance as I saw a similar fixup in comment.
From comments, I think this is needed, but I've not hit any issues due
to this.

gcc/ChangeLog:

	* config/riscv/predicates.md (prefetch_operand): mack 5 bits.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>
(cherry picked from commit b960201091fcab631a34a8c8d5b30e9f297dfbe5)