riscv: Simplify BUILTIN_DTB device tree mapping handling

__PAGETABLE_PMD_FOLDED defines a 2-level page table that is only used in
32-bit kernel, so there is no need to check for CONFIG_64BIT in #ifndef
__PAGETABLE_PMD_FOLDED and vice-versa.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>