riscv: fixup max_low_pfn with PFN_DOWN.

[ Upstream commit 28198c46 ]

max_low_pfn should be pfn_size not byte_size.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Signed-off-by: Mao Han <mao_han@c-sky.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>