RISC-V: Introduce sv48 support without relocatable kernel

This patchset allows to have a single kernel for sv39 and sv48 without
being relocatable.

The idea comes from Arnd Bergmann who suggested to do the same as x86,
that is mapping the kernel to the end of the address space, which allows
the kernel to be linked at the same address for both sv39 and sv48 and
then does not require to be relocated at runtime.

This implements sv48 support at runtime. The kernel will try to boot
with 4-level page table and will fallback to 3-level if the HW does not
support it. Folding the 4th level into a 3-level page table has almost
no cost at runtime.

Note that kasan region had to be moved to the end of the address space
since its location must be known at compile-time and then be valid for
both sv39 and sv48 (and sv57 that is coming).

* riscv-sv48-v3:
  riscv: Explicit comment about user virtual address space size
  riscv: Use pgtable_l4_enabled to output mmu_type in cpuinfo
  riscv: Implement sv48 support
  asm-generic: Prepare for riscv use of pud_alloc_one and pud_free
  riscv: Allow to dynamically define VA_BITS
  riscv: Introduce functions to switch pt_ops
  riscv: Split early kasan mapping to prepare sv48 introduction
  riscv: Move KASAN mapping next to the kernel mapping
  riscv: Get rid of MAXPHYSMEM configs

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>