usb: common: introduce of_usb_get_maximum_speed()

this helper will be used for controllers which
want to work at a lower speed even though they
support higher USB transfer rates.

One such case is Texas Instruments' AM437x
SoC where it uses a USB3 controller without
a USB3 PHY, rendering the controller USB2-only.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Change-Id: Ibf113325842caf72355707f7b44f29a55cd931b9
Git-commit: 1494a1f62bf7cf57345e9282c8189fe2a21fab64
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>