RISC-V: ignore xipImage

This built file shows up in "git status" without an explicit ignore.
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20220420184056.7886-3-palmer@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/boot/.gitignore

@@ -4,3 +4,4 @@ Image.*
 loader
 loader.lds
 loader.bin
+xipImage