RISC-V: Setup init_mm before parse_early_param()

We should setup init_mm before doing parse_early_param() in setup_arch()
to be consistent with setup_arch() of other architectures such as x86,
ARM, and ARM64.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>