Enable the EHCI generic driver, which is superior to ad-hoc SoC specific one.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>