riscv: Do not restrict memory size because of linear mapping on nommu

riscv: Do not restrict memory size because of linear mapping on nommu

It makes no sense to restrict physical memory size because of linear
mapping size constraints when there is no linear mapping, so only do
that when mmu is enabled.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/linux-riscv/CAMuHMdW0bnJt5GMRtOZGkTiM7GK4UaLJCDMF_Ouq++fnDKi3_A@mail.gmail.com/
Fixes: 3b6564427aea ("riscv: Fix linear mapping checks for non-contiguous memory regions")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20240827065230.145021-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>