RISC-V: Provide pgtable_l5_enabled on rv32

A few of the other page table level helpers are defined on rv32, but not
pgtable_l5_enabled.  This adds the definition as a constant and converts
pgtable_l4_enabled to a constant as well.

Link: https://lore.kernel.org/r/20230830044129.11481-2-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>