riscv: Move the linear mapping creation in its own function

riscv: Move the linear mapping creation in its own function

No change intended, it just splits the linear mapping creation from
setup_vm_final: this prepares for upcoming additions to the linear
mapping creation.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20230324155421.271544-3-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>