UCC28070: can ucc28070 support 1kHz AC input

Hi Qiu:

We had tested UCC28070EVM at 800Hz, it can work normally but a little worse of PF.  If you want to further increase the input frequency to 1Khz, it will further  influence the PF and ITHD,and should be normal working.

attached is the 800Hz test report.1452.3162.UCC28070_800Hz.pptx