ARM: UniPhier: move uniphier_ehci_reset() function

Because uniphier_ehci_reset() is only called from ehci-uniphier.c,
it can be a static function there.

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