riscv: mm: init: Consolidate vars, functions

Consolidate the following items in init.c

Staticize global vars as much as possible;
Add __initdata mark if the global var isn't needed after init
Add __init mark if the func isn't needed after init
Add __ro_after_init if the global var is read only after init

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>