MAINTAINERS: bpf: Add Lehui and Puranjay as riscv64 reviewers

Lehui and Puranjay have been active RISC-V 64-bit BPF JIT
contributors/reviewers for a long time!

Let's make it more official by adding them as reviewers in
MAINTAINERS.

Thank you for your hard work!

Signed-off-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/r/20240405123352.2852393-1-bjorn@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>