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.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>