riscv: mm: init: try best to use IS_ENABLED(CONFIG_64BIT) instead of #ifdef

riscv: mm: init: try best to use IS_ENABLED(CONFIG_64BIT) instead of #ifdef

Try our best to replace the conditional compilation using
"#ifdef CONFIG_64BIT" by a check for "IS_ENABLED(CONFIG_64BIT)", to
simplify the code and to increase compile coverage.

Now we can also remove the __maybe_unused used in max_mapped_addr
declaration.

We also remove the BUG_ON check of mapping the last 4K bytes of the
addressable memory since this is always true for every kernel actually.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>