usb: dwc2: make otg clk optional

Fixes commit 09a75e85
("usb: dwc2: refactor common low-level hw code to platform.c")

The above commit consolidated the low-level phy access into a common
location. This change made the otg clk a requirement and broke some
platforms when it was moved into platform.c.

So make clk handling optional again.

Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: 09a75e85 ("usb: dwc2: refactor common low-level hw code to platform.c")
Signed-off-by: Felipe Balbi <balbi@ti.com>