MIPS: Alchemy: use the OHCI platform driver

Convert the Alchemy platform to register the ohci-platform driver, now that
the ohci-platform driver properly handles the specific ohci-au1xxx resume
from suspend case.

This also greatly simplifies the power_{on,off} callbacks and make them
work on platform device id instead of checking the OHCI controller base
address like what was done in ohci-au1xxx.c.

Impacted defconfigs are also updated accordingly to select the OHCI platform
driver.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>