usb: Remove unused ppc4xx EHCI host driver

ppc4xx support was removed some time ago. Lets remove the now unused
EHCI driver and all its references for this platform as well.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Marek Vasut <marex@denx.de>