usb: dwc3: omap: add dwc3_omap_readl/writel functions

We separate between dwc3-omap helper functions to dwc3-core helper
functions. This will allow us to change the helper functions
implementation according to each module need.

Signed-off-by: Ido Shayevitz <idos@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>