riscv: Remove redundant restriction on memory size

The original reason for reserving the top 4GiB of the direct map
(space for modules/BPF/kernel) hasn't applied since the address
map was reworked for KASAN.

Signed-off-by: Stuart Menefy <stuart.menefy@codasip.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240624121723.2186279-1-stuart.menefy@codasip.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>