The LS1088a USB controllers are the same as LS1043a or LS1046a USB controllers. See the Section 3.13.4 of the LS1043 Datasheet:
USB 3.0 SSPHY needs a reference clock. There are two options for the reference clock: SYSCLK or DIFF_SYSCLK/DIFF_SYSCLK_B.
Have a great day,
Pavel Chubakov
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