ARM: uniphier: switch over to generic EHCI driver

The UniPhier EHCI driver (drivers/usb/host/ehci-uniphier.c) does
nothing special but set the base address and handle reset/clock.

Since commit 4feefdcf ("usb: add clock support for generic EHCI"),
the generic one (drivers/usb/host/ehci-generic.c) can do those, too.

We no longer need to stick to the dedicated driver.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>