[RISCV][NFC] Use named arguments in newly added changes

Named arguments have been supported in D152998 and most *.td files
of RISC-V target have been rewritten. This patch just makes them
consistent since some newly added changes weren't in this way.

Reviewed By: 4vtomat, MaskRay, asb

Differential Revision: https://reviews.llvm.org/D157580