if_dwc: recognize additional rgmii phy-modes

Per the reports, some Allwinner device trees now list the desired
phy-mode as "rgmii-id". The manual string comparison fails to detect
this, and we end up falling back to MII mode. Instead, select the clock
name using the sc->phy_mode variable, which is set in the main attach
function.

The logic to actually handle rgmii-id mode delays will be added to the
relevant PHY driver.

PR: 261355, 264673
Reported by: Maren <marentoy@protonmail.com>
Reported by: Arie Bikker <src-2016@bikker.homeunix.net>
Reviewed by: manu
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35500