riscv: Get rid of MAXPHYSMEM configs

CONFIG_MAXPHYSMEM_* are actually never used, even the nommu defconfigs
selecting the MAXPHYSMEM_2GB had no effects on PAGE_OFFSET since it was
preempted by !MMU case right before.

In addition, the move of the kernel mapping at the end of the address
space broke the use of MAXPHYSMEM_2G with MMU since it defines PAGE_OFFSET
at the same address as the kernel mapping.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: 2bfc6cd8

 ("riscv: Move kernel mapping outside of linear mapping")
Signed-off-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Conor Dooley <Conor.Dooley@microchip.com>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>