MIPS: Alchemy: use the ehci platform driver

Use the ehci platform driver power_{on,suspend,off} callbacks to perform the
USB block gate enabling/disabling as what the ehci-au1xxx.c driver does.
Update the db1200 and db1300 defconfigs to now select the EHCI 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>