linux-imx

riscv: turn mm_segment_t into a struct

This matches what other heavily used architectures do, and will allow us
to easily use <asm-generic/uaccess.h> for the nommu case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>