Added MTLGPUFamilyApple6 and MTLGPUFamilyApple7 caps.

Bug: angleproject:2634, angleproject:5672
Change-Id: Icc35f823cdd018246a8ac3b1179adc81584fb204
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2846832Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Le Hoang Quyen <le.hoang.q@gmail.com>
Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>