Update size attribute for Power10.

Update size attribute for Power10.

2021-03-03  Pat Haugen  <pthaugen@linux.ibm.com>

gcc/
	* config/rs6000/dfp.md (extendddtd2, trunctddd2, *cmp<mode>_internal1,
	floatditd2, ftrunc<mode>2, fix<mode>di2, dfp_ddedpd_<mode>,
	dfp_denbcd_<mode>, dfp_dxex_<mode>, dfp_diex_<mode>,
	*dfp_sgnfcnc_<mode>, dfp_dscli_<mode>, dfp_dscri_<mode>): Update size
	attribute for Power10.
	* config/rs6000/mma.md (*movoo): Likewise.
	* config/rs6000/rs6000.md (define_attr "size"): Add 256.
	(define_mode_attr bits): Add DD/TD modes.
	* config/rs6000/sync.md (load_quadpti, store_quadpti, load_lockedpti,
	store_conditionalpti): Update size attribute for Power10.