[AArch64] [Obvious] Correct pattern target requirement

[AArch64] [Obvious] Correct pattern target requirement

Had mistakenly used a target macro that was not defined and not the
relevant one instead of the macro that should be used.

TARGET_ARMV8_6 is not defined, and also not the macro we want to check.
Instead check TARGET_F64MM.

gcc/ChangeLog:

2020-01-17  Matthew Malcomson  <matthew.malcomson@arm.com>

	* config/aarch64/aarch64-sve.md (@aarch64_sve_ld1ro<mode>): Use
	the correct target macro.