nvptx: Back-end portion of a fix for PR target/104489.

nvptx: Back-end portion of a fix for PR target/104489.

This one line fix/tweak is the back-end specific change for a fix for
PR target/104489, that allows the ISA for GCC's nvptx backend to be bumped
to sm_53.  The machine-independent middle-end pieces were posted here:
https://gcc.gnu.org/pipermail/gcc-patches/2022-February/590139.html

2022-02-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/104489
	* config/nvptx/nvptx.md (*movhf_insn): Add subregs_ok attribute.