riscv: s64ilp32: Add MMU_SV32 mode support

This needs to add Sv32 mode in the SATP CSR of RV64 ISA, a novel
extension of 64-bit processors' MMU. It could save a bit of page
table footprint and improve the page table walk performance:

s64ilp32 with Sv39:
PageTables:          136 kB

s64ilp32 with Sv32:
PageTables:           60 kB

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>