ARM: UniPhier: remove EHCI platform devices

Now UniPhier platform highly depends on Device Tree configuration
(CONFIG_OF_CONTROL is select'ed by Kconfig).  Since the EHCI is only
used on main U-Boot, we can drop platform devices of the EHCI
controllers.  We still keep UART platform devices because they might
be useful for SPL.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Marek Vasut <marex@denx.de>