riscv: add support to check machine length at runtime

Highest two bits of misa can be used to check machine length. Add code
to support this.

Change-Id: I3bab301d38ea8aabf2c70437e179287814298b25
Signed-off-by: Xiang Wang <wxjstz@126.com>
Reviewed-on: https://review.coreboot.org/27770
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
1 file changed