RISC-V: pass arg6 in sbi_call

Allow for an additional argument to sbi_call which will be passed in a6.
This is required for SBI spec 0.2 support, as a6 will indicate the SBI
function ID.

While here, introduce some macros to clean up the calls.

Reviewed by: kp, jhb
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D22325