USB EHCI: reset root hub

Some of multi-function USB controllers (e.g. ISP1562) allow root hub
resetting only via EHCI registers. So, this patch adds the
corresponding kind of reset to OHCI's hc_reset() if the newly
introduced CONFIG_PCI_EHCI_DEVNO option is set (e.g. for Socrates
board).

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>
Acked-by: Markus Klotzbuecher <mk@denx.de>