ARM: SAMSUNG: Remove unused s5p_device_ehci

Since commit ca914350 "ARM: EXYNOS: Remove unused board files",
s5p_device_ehci is not used anymore. Thus, s5p_device_ehci can
be removed. Also, unnecessary S5P_DEV_USB_EHCI option is removed.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>