C-SKY: Support fldrd/fstrd for fpuv2 and fldr.64/fstr.64 for fpuv3.
gcc/ChangeLog:
* config/csky/csky.c (ck810_legitimate_index_p): Support
"base + index" with DF mode.
* config/csky/constraints.md ("Y"): New constraint for memory operands
without index register.
* config/csky/csky_insn_fpuv2.md (fpuv3_movdf): Use "Y" instead of "m"
when mov between memory and general registers, and lower their priority.
* config/csky/csky_insn_fpuv3.md (fpuv2_movdf): Likewise.