feat: use x0-x17 for ff-a calls

Update ff-a calls to use all registers from x0-x17 as input and as
output. Previously, only x0-x7 were being used.

Signed-off-by: Raghu Krishnamurthy <raghu.ncstate@gmail.com>
Change-Id: I52e9f4749578f5e6265dff9beb6b0520dcc72bc5
1 file changed