riscv: Add macro for multiple nop instructions

Some cases need multiple nop instructions and arm64 already has a
nice helper for not needing to write all of them out but instead
use a helper to add n nops.

So add a similar thing to riscv and convert the T-Head PMA
alternative to use it.

* 'riscv-nops' of git://git.kernel.org/pub/scm/linux/kernel/git/palmer/linux.git:
  riscv: convert the t-head pbmt errata to use the __nops macro
  riscv: introduce nops and __nops macros for NOP sequences