riscv: mm: Update the comment of CONFIG_PAGE_OFFSET

Since the commit 011f09d1 set sv57 as default for CONFIG_64BIT,
the comment of CONFIG_PAGE_OFFSET should be updated too.

Fixes: 011f09d1

 ("riscv: mm: Set sv57 on defaultly")
Signed-off-by: Song Shuai <suagrfillet@gmail.com>
Link: https://lore.kernel.org/r/20230809031023.3575407-1-songshuaishuai@tinylab.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>