riscv: correct pt_level name via pgtable_l5/4_enabled

The pt_level uses CONFIG_PGTABLE_LEVELS to display page table names.
But if page mode is downgraded from kernel cmdline or restricted by
the hardware in 64BIT, it will give a wrong name.

Like, using no4lvl for sv39, ptdump named the 1G-mapping as "PUD"
that should be "PGD":

0xffffffd840000000-0xffffffd900000000    0x00000000c0000000         3G PUD     D A G . . W R V

So select "P4D/PUD" or "PGD" via pgtable_l5/4_enabled to correct it.

Fixes: e8a62cc2

 ("riscv: Implement sv48 support")
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Song Shuai <suagrfillet@gmail.com>
Link: https://lore.kernel.org/r/20230712115740.943324-1-suagrfillet@gmail.com
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230830044129.11481-3-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>