riscv: Start of DRAM should at least be aligned on PMD size for the direct mapping

So that we do not end up mapping the whole linear mapping using 4K
pages, which is slow at boot time, and also very likely at runtime.

So make sure we align the start of DRAM on a PMD boundary.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reported-by: Song Shuai <suagrfillet@gmail.com>
Fixes: 3335068f ("riscv: Use PUD/P4D/PGD pages for the linear mapping")
Tested-by: Song Shuai <suagrfillet@gmail.com>
Link: https://lore.kernel.org/r/20230704121837.248976-1-alexghiti@rivosinc.com


Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>