Documentation: kdump: Add va_kernel_pa_offset for RISCV64

Documentation: kdump: Add va_kernel_pa_offset for RISCV64

RISC-V Linux exports "va_kernel_pa_offset" in vmcoreinfo to help
Crash-utility translate the kernel virtual address correctly.

Here adds the definition of "va_kernel_pa_offset".

Fixes: 3335068f8721 ("riscv: Use PUD/P4D/PGD pages for the linear mapping")
Link: https://lore.kernel.org/linux-riscv/20230724040649.220279-1-suagrfillet@gmail.com/
Signed-off-by: Song Shuai <suagrfillet@gmail.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230724100917.309061-2-suagrfillet@gmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>