riscv: Use p*d_leaf macros to define p*d_huge

The newly introduced p*d_leaf macros allow to check if an entry of the
page table map to a physical page instead of the next level. To avoid
duplication of code, use those macros to determine if a page table entry
points to a hugepage.

Suggested-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>