phy: lantiq: rcu-usb2: wait after clock enable

phy: lantiq: rcu-usb2: wait after clock enable

Commit 65dc2e725286 ("usb: dwc2: Update Core Reset programming flow.")
revealed that the phy isn't ready immediately after enabling it's
clocks. The dwc2_check_core_version() fails and the dwc2 usb driver
errors out.

Add a short delay to let the phy get up and running. There isn't any
documentation how much time is required, the value was chosen based on
tests.

Signed-off-by: Mathias Kresin <dev@kresin.me>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: <stable@vger.kernel.org> # v5.7+
Link: https://lore.kernel.org/r/20210107224901.2102479-1-dev@kresin.me
Signed-off-by: Vinod Koul <vkoul@kernel.org>